No, that is exactly right.
If we look at what is common custom in all kinds of sports, Mayweather is unbelievably lucky that boxing has weight classes. Outright he is far away from even belonging to the best boxers in the world, because boxer from heigher classes would easily knock him out.
Now I understand that he can be one of the best just for technical aspects, but in other sports that is irrelevant, because anyone says that having the perfect body for the sport is part of being great. That’s why I brought up Schwartzman, because he is so small that he has decisive disadvantages in tennis (especially due to a weak serve). So if tennis would be like boxing, there would probably be heigth classes.
If Schwartzman then dominates let’s say the class of people from 1.67 m to 1.72 m clearer than Federer or Nadal their heigth class, then with the same logic he would have to be called GOAT as if someone calls Mayweather GOAT for totally dominating a weight class.
You said Schwartzman has never won anything, but surely he would if tennis would be organized like boxing. It is very reasonable to make this comparison. If anything, he is nearer to the absolute top in his sport than Mayweather.
Also remember in boxing the competition of course is extremely small if every 3-5 kg a new weight class begins, and that’s the case in the lighter classes. I just wanted to make clear how laughable it would be to call someone like Mayweather GOAT.
Imagine other sports (for example tennis) being divided in 15-20 divisions, and each of them not crowning one champion, but up to 4.