It's not entirely cheating, but it WILL cause misunderstandings and look questionable. Like in this case.
Like...yeah, there are some friendships where you joke about dating each other and whatnot, and your spouses can be in on the joke. Hell, my mom has a "Facebook" husband (granted, he's gay and happily married), but it's a joke. She's not attractive to him at all, and she made that clear. But here...it doesn't sound like he ever gave her context to this joke?
It sounds like she found out, and he immediately was like "oh no, this is just a joke". Which I can only guess made him look questionable. Then again, the added context is also that he tends to spin the truth, so there may or may not have been more going on and maybe it really wasn't a joke.
There's also the ex-girlfriend's reaction, which (at least for me) implies that she's been cheated on before and any sign (joke or not) gives her a red flag.
Like...sometimes, people can joke about that in a smaller group that knows everyone and knows it's not for real. But in this case, it only caused a misunderstanding and just made him questionable.
Can't really say he didn't deserve it.