I agree with that.
If you can't compare ERA's then how can one be rated #1?
It's an old argument that will not die, but in the process you pick up a few things here and there about tennis so it's not a waste of time.
Thing is we are comparing achievments and not really who is the best tennis player of all time. If we ask the question about the BEST, we need to ask more questions like the best for a day, a year or a career? Mcenroe 84 might be the best tennis ever played but he couldnt sustain that for more than 1 year. Best tennis I have seen is probably Federer 2006 but Laver 69 with wooden raquet might be the best you can play with wooden raquets. Anyway, it is good to seperate personal opinions from an objective criteria to find a "GOAT", personal opinions can be an interesting part of another discussion but this list is very objective and very fair to players of different eras and that makes it very easy to compare and keep track on where modern players are ranked in terms of achievment.
Certanly you can still claim Federer has achieved more than Laver back in his days, but it should not be in terms like "players back then were not as good" and therefor every slam today is worth more than back in Laver's day. You can question if an amateur slam and pro slam should be counted the same as an open era slam, that is a fair critisism to the criteria of the list.