Any player can concede for any reason; you just have to learn who throws a hissy fit and who doesn’t. I’ve had an Allied player forfeit on me after UK ROUND ONE. Yep, a scramble didn’t go his way, and a couple of bad rolls in the Med and claimed there was “no way back”. Meh, a win’s a win, but I’m still in it for the long haul with expectations of an epic game.
In general, a sucessful Sealion doesn’t an Axis victory make. I’ve lost as the Axis after a sucessful Sealion; the Russian player knew what to do and put immense pressure on a front stripped-bare for landing units. If Japan gets some bad luck and isn’t threatening victory, it’s bad.
That said, i’ve won plenty of times after a good Sealion, so it’s not an unheard of response.