True. One could argue that when we talk about the best footballers ever we don't specifically say they are male. Just the best.
Pele never played against a women's team.
This is why sensible people always talk about the 'best pound-for-pound' fighter in the world rather than making silly, meaningless statements that ignore the reality that a tiny man like Mayweather wouldn't even scratch most heavyweights and would very likely be killed were one to land a blow on him.
So must it be with female tennis players. Male tennis players are demonstrably better players as they would beat their female equivalents. Thus, a woman cannot be the best tennis player in history by definition.
Why is it a cop out? It's simply an acknowledgement that it would be absurd to say that Sugar Ray Robinson was a better fighter than Muhammad Ali since - had they ever fought - Ali would almost certainly have won due to his greater size, weight, strength and reach.