Clemson may not be as good as advertised, but so was USC in '17. The difference is that, despite playing nobody up to that point, USC already had a loss to Washington State.
Clemson made it's way through the maligned ACC unscathed to this point, whereas USC failed to do the same in an arguably worse Pac-12 slate.
I think the other thing that puts this game over the top is that this game is one to build off. Nobody was truly buying Darnold and Helton, and they were still outside the top 10 when ND de-cleated them. Clemson was one of the teams that ND had to strive to beat in order to reach the next level.
And up to this point, ND had gone 2-22 in the past 24 matchups against top 5 opponents. Those wins were against Michigan (who finished 7-5) and Clemson without their starting QB, who then proceeded to whoop us in the ACCCG. This game will most likely stand on it's own as a significant blowout victory over a top opponent, maybe not a playoff caliber one, but certainly one of the better programs in CFB.
It was also huge for building credibility for Marcus Freeman, who needed every bit of it.