The air in the tank loses heat to the tank walls by conduction, which then lose heat to the surrounding air by either conduction or radiatively if the tank is painted.
A non-radiating planetary atmosphere cools by setting up planetary scale circulations where warm air rises at the equator, cooling adiabatically as it rises, then is driven poleward by continuing heating of the gas below it. Gas flows towards the equator at the surface, heating along the way, until it rises at the equator. The Cold air aloft then sinks at the poles (or more poleward latitudes), where the surface temperature is colder since the net solar flux is less (it's slanted with respect to the incoming shortwave). The gas heats adiabatically since as it descends it gets compressed, and it's temperature is now higher than the surface temperature. So it conducts heat back to the surface, which then radiates it to space, maintaining net radiative equilibrium.
Don't believe me? Look up planetary scale circulations on Mars, which has a weak greenhouse effect and you will see that it looks exactly like I've described for the same reasons. Physics is physics.
There is no such thing as a perfect insulstor. A planet with an atmosphere of pure nitrogen and the same surface pressure as Venus could still achieve thermal equilibrium through conduction. With a poor conductor like nitrogen, such a process would take a long time, but a few billion years would be long enough.
But for the atmosphere to get hot in the first place would depend on how it got hot. Compressing a gss eill do it, but just becsude a gas is under high pressure is not the same as to say that the gas has bern compressed.
And in an atmosphere of pure nitrogen, a laps rate could mean a big difference in temperature between the upper atmosphere and the lower atmosphere, but that would make the upper atmosphere very cold. The gas at the surface will be the same temperature as the surface, which in turn be the same as if it hsd no atmosphere.
Your understanding is inaccurate. In the simplest form or example possible, heat is the excitation of molecules and atoms. Absolute zero, theoretically, where there is no motion or excitation of matter, what so ever. The one who answered that question is correct. Heat can be 'absorbed' by it's surroundings. It takes energy to keep those atoms and molecules excited. When pressure is applied to matter you have so much excitation in the uncompressed form and by compressing you take that same amount of excitation and put it into a smaller area. So when you do that you have 'condensed' energy which is hotter.
A better example would be that of an air conditioner. You take Freon and condense it with a compressor. This changes the state from a gas to a liquid and it becomes very hot. Why? Because you have taken the same amount of excitation and compressed it into a smaller area. Go outside your house when your AC is running and put your hand on that coil (Carefully, because it could burn you.) It is hot. That coil is called the condenser coil. But now you take a fan and blow outside ambient heat across that coil and at the end of the coil you have a liquid that the heat has been taken away from. Now you have a cooled liquid, with less excitation and when it goes through an expansion valve of some sort, whether it a TX or capillary type, it will release that pressure and in order for the Freon to change state it has to 'absorb' the heat around it, which is being supplied by the fan blowing the ambient room temperature through the 'A' coil and this comes out not as cool air but air with less heat.
So the person that answered the question, did so, appropriately. The 'ambient' temperatures of space are really cold and it is a vast heat sink. So think of Venus as a condenser coil with the compressor turned off. Just as the Freon will go back to equilibrium so would the atmosphere of Venus. It is not totally the same as you would have to turn off your expansion capabilities, but you can get the basic idea.
Conduction is a molecular transport phenomenon. Energy is transported by the kinetic energy of the molecules (including internal vibrations) molecules. Likewise, convection is a fluid dynamic property of a collection of molecules. Both processes are fast compared with radiative transport. A nitrogen atmosphere will radiate about 1e-12 as much as CO2 or H2O on a molecule by molecule basis and will hence cool slower due to radiation. However, if the planet has a rocky or liquid surface, the cooling will be faster because the atmospheric heat will be conducted to rock via molecular collisions and then radiated virtually unhindered to space.
Edit gcnp is right that pressure changes the absorption profile. My answer implicitly assumes Earth atmospheric pressure.
Nether gas would lose heat it would only be exchanged or transported until a equilibrium was reached. If a huge amount of nitrogen was suddenly released in Venus atmosphere and having a density less then CO2. That would create a upper movement, if it where sudden and temperature driven. That would create a mix striving for equilibrium.
Of course you can plot as many different scenarios as you wish. In simple English we call it blending. It's not a new concept.
Yes Venus is hot, not because of compression of gasses but because it's dense atmosphere acts as a 99.9% insulation blanket, all rocky planets were hot and molten when formed ( due to formation processes) Earth cooled down, and Mars cooled down a lot, Venus not so much because of it's dense atmosphere.
I have just read this in answer to another question:
"And the notion that Venus is hot because the pressure is high is absolute nonsense. Yes, air is hot when it comes out of a compressor, but the same air quickly cools down if it remains in the tank. If somehow, a huge amount of gas were suddenly dropped on a planet, and for simplicity, let's say the gas is nitrogen, which is not a greenhouse gas, the planet would get very warm. And then it would be in a state of radiative disequilibruim, until it cools back down."
How would it cool back down?
My understanding is that N2 does not radiate because its emissivity is so low. The air in the tank on earth can conduct and convect. The N2 on Venus cannot do that.
