I can't speak for his appeal but he was a popular LoL content creator during the game's heyday, and is now part of the same circle as Valkyrae, LilyPichu, Pokimane etc. He is a known V-Tuber fan (and not just Hololive) so this doesn't spring entirely out of the ordinary. I don't watch his content, but anyone who isn't a cunt or one of those Minecraft LPers who grift children is totally fine in my book.
It's worthy of note, though, that these kinds of collabs ARE the Nijisanji way. Their JP branch very often collabs with high-end seiyuu and actors, mangaka, popular YouTubers, and professional gamers. As the EN branch grows, the likelihood that you are going to see them with big-name YouTubers and Twitch thots, as well as the woke sect of anime VA's, grows with it.
It's just the reality of things. Not necessarily going to make for the most exciting content, but it will give them an absolute fuckload of connections and opportunities they may need to fight the competition on other fronts that aren't sheer numbers.
NijiEN will probably always be competing for 2nd Place, but they do have the talent and facilities to keep on chipping away at other companies' slices of the pie.