I’m surprised Ohtsuki and Hiroi’s story like this never prominently show up in the main series. This fleshes out their relationship and characters a bit more, plus we get to see why Hiroi is adored by Ohtsuki beyond her musical skills.
In short, this is a nice chapter that can slot in the main series IMO, though yes it’s outside of the main group.
Also, I’m willing to bet Ohtsuki paid for Hiroi’s alcohol when they had karaoke.