This is pretty true actually
I've seen Sampras fans here over the years claim that Novak is better than Fed despite all of his Wimbledon victories being against a 30+ year old Fed.
Of course when then those same Sampras fans are confronted by the argument of 29 year old defending champion Pete losing to teenage Fed (in a pretty high quality old-school grasscourt match BTW, Pete had 70% 1st serves in), they completely excuse that loss away and put it down to Pete being old (without even blinking lol). Sports fandom, what can you do, double standards are the norm.
That said I do think Novak's underrated on grass by Fedal TTW fandom to be honest. At his best, his serve/return combo, sheer athleticism (which is something Agassi lacked and why he wasn't a bigger threat to Sampras) and underrated serve +1 make him one tough customer on grass. Handling someone like 2015 Fed (not prime Fed but still very dangerous, as
Murygoat can attest after getting slaughtered in the semis) in almost routine fashion was pretty impressive in my book, can't do that if you're not great on grass. He's not Pete/Fed level but he's legit great on grass, and yes better than the Nadal.