While what you said probably was part of the reason, I think Kasumi also doesn't want to acknowledge Tatsuya. From the beginning, she has been having irrational hate towards him. Even after being told about his accomplishment, she was still sus about him.
I think she acknowledges his strength and abilities to some level especially since he's her senpai, but that doesn't make her like him. And their first meeting wasn't the best. I think she just dislikes him due to how Mayumi was glued to him.
Maybe she thinks he's being a smartass, playing a role of an ideal magician student trying to pick up girls that way while he's probably no different than any other, maybe just above an average but nonetheless still having some flaws to him.
I think she's saying more of 'we've never lost control of our magic before' in counter to him saying the two doesn't have enough fine control to prevent their double storm from going out of control.
Given their flinching upon him pointing that out, I think the two are self-aware of that issue. But Kasumi believe Tatsuya's call was premature (either that the two were still in control enough not to break the rule and thus only Shippou deserve the disqualification, or that she think they could've taken down Shippou before things get that bad)
So she's immature but not without basis, which is probably why Tatsuya was willing to have a match with them to prove that point.
Compare to Shippou who's made so many mistakes and just try to make excuses that Tatsuya don't even care to deal with him.
I think the fact that they may have already started to lose control over it but failed to notice it it the sign of them being immature. It's not about having a basis. He's a grade upper and he dealt with your trump card in a second while the other guy struggled with it. She's just being upset about him being able to deal with their trump card which gives them advantage over your average magicians and Tatsuya willing to have a match with them was to help her cool down and possibly cause a situation where they in fact lose control over it.
It's not like they can kill me anyway so I won't lose anything is probably his mentality about it.