Sounds like you are overly discounting the talent in the ACC. Sure, overall the ACC, especially the top teams, do not have the depth of talent that the SEC or B10 have. But there are competitive teams in the ACC beyond Clemson.
It would seem that PFF rates players on talent and level of play, maybe looking somewhat ahead to what each player may end up being rated as NFL talent as well. But their rating numbers, by game and over a season, are based upon making plays at their position. Regarding Allar, I've seen him as projected to be going in the low end of the first round where as the Clemson QB Klubnick is projected as the #1 overall pick. So who would you want at QB for your team late in the 4th quarter with the game on the line, Allar or Klubnick? NFL evaluators and PFF as well would want Klubnick and I agree with them based upon what Allar has done against good teams with the game on the line, throwing abysmal picks against Oregon and ND when he had the chance to shine at crunch time.
Lots of B10 teams would get pushed around by Clemson as well.