Once I get to know someone, I gak-talk with them just 'cuz. For my close gaming group, it's just the done thing.
We've played games so long, that there's no "real" ego in it. You have good, bad, fun, slog, all kinds of games. The reward in winning is lording it over everyone else until you get beaten next. Or cutting away from their victory by pointing out how their victory is hollow, until you win.
Turn a total fluke win into a story of epic genius.
You had a flawed plan from the get go? He's playing Eldar, it didn't matter what you did in the first place, dirty pointy-ear. Playing on easy-mode he is.
For me, it's part of the social after-party. Re-establishing the pecking order of who's good at what. I think that for us, it's like other people gambling on sports. The result of the game is interesting, but it gives us a "prize" to play for. A momentary position of being king of pebble mountain, but king none-the-less. Don't you forget it.
To that end... not everyone gets that. Not being accusatory, but it is my experience in playing with people on the spectrum, that they don't get "verbal rough-housing", and tend to take it personally instead of good-naturedly. I've encountered a large number of such persons, over the years. I adapted to that, and changed the way I engaged in that by actively letting them know it's just good natured fun, and they're welcome to swing back. I put the kid gloves on, but it still lets me play back and forth after the game.
I'd suggest trying to hit back. If your opponent is being "mean" about it, they'll likely sulk, and then you'll know. If they redouble the effort, and start making wilder claims... you know you're just messing around.