I’m not sure what your point is. We agree on the facts, and my speculation is as good as yours. There are some options a stateless person stuck in an airport without passport can pursue, but they take time, sometimes a whole lot of time, maybe even a lifetime (there’s even a movie about it, and a famous person it happened to) and in that time, does anyone actually think he was just treated normally and in good faith by Russian authorities, and he decided:

a) “hey these Russian folks are alright and want to help me out and certainly don’t spy on their own citizens like the US does which is famously what I was whistleblowing about in the first place maybe would like to live here forever because I don’t care about having human rights anymore”

and/or

b) “aha the US fell perfectly for my sinister plan to create plausible deniabilty to pretend that I wasn’t secretly a russian agent all along by making it look like I became trapped in Russia entirely by accident, when it was exactly where I wanted to be! those suckers!”

I don’t think either of those things are plausible. I think he got stuck in Russia, and the Kremlin realized how fortunate this was for them, and they took advantage of his stateless status to make sure he never left while making sure he said all the right things to make sure everyone knew it was entirely “his choice made of his own free will”, as such dictatorships like to do. I’m sure it has been explained to him that it would be dangerous for him to leave Russia’s “protection”. It’s speculation, granted, but I believe it in absence of any other more convincing theory.