Yonkou was talking about a quote from a manga/interview he read, I'm not sure where, that basically said (I'm paraphrasing here) that shounen manga have a common trope of "overcoming" the difficult situation they're in (i.e. Goku defeating Frieza in DB), and seinen have the trope of "learning to live within the problem" (i.e. Guts gaining new comrades and finding new meaning in his life after all his friends died and he and Casca went through trauma). I can't seem to find the original post where the talked about it and the mangaka he was quoting from like a month ago, but here's him explaining it today.
It obviously doesn't work ALL the time, but quite a lot of manga are like that. I mentioned that a lot of seinen are basically shounen but with more sex or gore.
Some people seemed to interpret it as him saying HXH wasn't like, good, or something? But he said he didn't mean being shounen was bad or anything. He meant that HXH is also an "overcome the problem, seize victory, defeat the enemy" type of story. Not that it's bad.