There's so many things wrong with that argument it's hard to know where to start. Suffice to say, if you're older than 12, it's embarrassing.
1. Take Rod Laver (or a Hoad, Rosewall, Gonzalez, Kramer, Budge, etc), with his championship mentality, and give him the same benefits as today's players and Laver would still be a champion. Unlike Federer he was never dominated by another player. That not only points to him having a better all-around game it points to him having a significantly stronger mindset.
2. You can't say 'oh, but he'd be too small to compete against today's athletes' BECAUSE, were Rod Laver to exist today he would be subject to the same things as everyone else. In other words, Rod Laver born in this era would not be a 5`8, he'd be about 6ft mark.
3. You can't say I'll take player X from his era, force him to use the same equipment as back then but compete against people user more efficient gear. That's just stupid. All it does is prove that today's racquets and string are more effective and efficient than the gear used previously and today's technique has changed to accommodate them. Even someone with a two digit IQ knows that to be true. However, it makes absolutely no comment on the ability of the player because you'd get exactly the same result if you took a player from today and forced them to use the same equipment.
Simply put:
1. What separates a great player from a good player is their championship MENTALITY.
2. What separates a person who understands professional sport from one who doesn't is the recognition of the first point.