I can kinda understand the monk being an additional recruit from that joinable sect, but was surprised the other since his biggest appearance before this was in an antagonistic role regarding another sect the player can befriend (but not join).

If you try and do that sect's questline while he's in the party does he suddenly go "Oh ♥♥♥♥ gais, gotta run, important business!"
Also it seems like there would be new dialogue added related to that whole scenario due to the changes? Depending on what content you'd done first.