Just watched the ending again from your link. It’s hard to see, but Child’s breath is visible near the end before he drinks from the bottle. I think the lighting hides his breath before then.
I do think the bottle that Child’s drank from contained gas. In that moment MacReady knew that Child’s was the Thing, and he let out a quiet chuckle.
i agree. it’s just a fan theory (not mine). in the videogame, you find Childs’ frozen body. it was a fun game to play when it came out for the original Xbox (it included an early “trust/mistrust” system which harkened back to the movie), although of course the graphics look dated now. you can follow the “official” story by watching the walkthrough videos on youtube.
https://www.youtube.com/watch?v=R62qyeNsIAM