Well, we already knew that the girls were a bit deluded from the reason they gave for switching vanguard when they're the ones who couldn't keep up with him and had asked him to hold back. They've probably grown stronger also, but they don't know he's not just stronger but can use magic. It's not like they've done anything bad besides being cocky and they indeed can't really vouch for his strength because they basically immediately asked him to hold back so they could participate and they didn't have his stamina to keep diving deeper in the dungeon.
I hope it'll be a quick 2 page duel where he blows back the knight, grabs them and they surrender. Then he can get back to hunting treants in the dungeon. No need to really waste time on them, an they'll have a reality check on how strong he actually is.