I mean, if they're so obsessed Kotaro and he just dropped that he's in love with a vtuber, wouldn't the first thing you'd do is to look and see what they're like to get a better idea? They all just found out and were like, "Oh, okay, then." with no curiosity at all.
Also, Jesus Fucking Christ, Shigure, just how thirsty are you? "His hands because they're big and make me feel good"? Then again, considering she admits to schlicking to Kotaru all the time and even did it all night before the test... I guess it's not a surprise.