It's not just that the buck stops at BK, it was his decision to hire and retain those two:
Brian Kelly hired Del Alexander, not Tommy.
Brian Kelly hired Jeff Quinn, not Tommy.
Brian Kelly was the one that failed to hold either of them accountable when they failed to land quality recruits at their position.
Keep in mind, this all PREDATES Tommy's promotion to OC. He took over in 2020. So pretty much all the players that Tommy had more involvement with are true sophomores or freshman currently.
By the time Tommy had even close to a full hold of the offensive staff, what was he going to do? Damage was already done. Quinn and Del had already screwed up multiple classes. And if Kelly wasn't going to move on from them after that, why would he listen to the rookie OC?