He basically has a point here. Pound for Pound literally means what it says, as Wilder only weighed 83%, 84½% and 86% of what Fury came in on, which is comparable to a Super Featherweight fighting a Light Middleweight. Of course Heavyweight has to be looked at in a different way, certainly with Fury actually just being obese half of the time, which only hinders him. But seen in the way it was meant when people came up with it, the op is actually right. P4P is nonsense anyway, because it's just wild guessing without taking all factors into account.
Pound-for-pound was certainly not invented for heavyweights. Being heavyweight king is the pinnacle of boxing.
Pound for Pound is a pointless concept made up by small men. The most important distinction in boxing is who is the best heavyweight.