To play Devil's Advocate for a moment, MC might have assumed that showing he is now on similar footing if not outright better than these three arrogant prodigies would throw them off-balance. At which point que the gloating, taunting about possibly losing to the talentless crybaby they smugly and effortlessly enjoyed steamrolling and humiliating in the past, before finally being stripped down to their birthday suits and as he stands over them sneering triumphantly. Them being girls likely doesn't change much, if anything, in his mind (being naked in-front of someone who looks down on you can be mortifying for everyone, regardless of gender). He is oblivious to their feelings for him at best, and subconsciously repulsed by them at worst (which is/would not be unreasonable or surprising given how they treated him in the past). And then there is the cherry on-top: his infatuation/simping for the v-tuber.
Of course, that's giving him (and this manga) as much credit as I can (and more than they both likely deserve lol). Like you, I see this match coming down to them being tied, both reduced to just their underwear (and being flustered by the other's appearance), the deciding point about to be settled...only to be interrupted by the prez at the last moment.