That is exactly how hot air balloons work, so despite the format of a meme it’s entirely boring and thus an antimeme, where part of the joke is that the poster betrayed the assumed relationship of them providing a joke.
It’s a reference to the “oil floats on water, wait for it to rain, cover yourself in oil, fly” meme. That meme is funny because of how stupid it is, so this meme partially inherits its stupidity through the reference. References are also funny.
The combination of an apparent antimeme and the inherited stupidity is kinda interesting, because we all know the oil flying method is stupid but the hot air method works. This highlights that the hot air method sounds equally stupid to someone whose never seen it in action. It’s funny that such a dumb meme could provide this insight.
I guess I don’t get the joke because I thought that’s exactly how hot air balloons worked
It’s a multilayered joke:
Oh.
This is the troll science format. Except it’s a double troll, because it is real science pretending to be troll science pretending to be real science.
Or you could interpret it as OP saying hot air balloon physics are so wacky, they sound made up.
It’s a Trollface anti-joke I guess.