Traps and ass on the bench is all that matters to be legit. I used to train with pretty well known PLer that could hold an arch like that when he benched.
Yeah I'm not sure if the arch might make it more difficult I've never seen that before.. Either way what a beast I only have 65 lbs on a 14 year old girl lol...
I read an article a few years ago discussing this. There is a reason why there are so many very young female athletes can bench what they do, and it's because of their extreme arches and stature that allows them to stay "legal" while doing so. It's all about biomechanics, physics and technicalities. Of course, it didn't go without saying, these young athletes still have to train hard and practice, so it didn't completely discredit them. This is also why progress is no longer linear after about 16 or 17