@trapsarebetter
Yeah, that's the weird part. It's just that usually, the manga artist is different from the light novel illustrator. However, Arumi as far as we've been shown is also another light novel illustrator.
Both Sagiri and Masamune are literally kids, so they wouldn't know about these stuff. At the moment, they only set up these requirements: illustrations that can rival Sagiri, loves their novel as much as Sagiri does. Sure, Arumi fits the criteria, but it seems that they forgot that they also have to be a manga artist.