Oh ffs.
I do not own XCom on Steam, so I cannot test that for you, but as I said, I have tested 5 other games, all of which activates through Steam and requires Steam to run. I can provide pics of the error messages I get when I try to run them without the client. If needed, I can go through my entire game collection and test them for you.
The case might be different for a game from 94 like XCom which is run through Dosbox, but you cannot use that as proof that Steam isn