Well I mean it is relative. Of course Alcarez is nowhere near Nadal on clay yet (probably never), but I am comparing him to Hewitt and Roddick who are far below contenders level on clay, particularly Roddick. Alcrarez is still by a huge margin better than both on clay already, regardless that he hasn't reached a RG final. And it is probably already a bigger gap than the gaps at Wimbledon and US Open combined, even though I agree with you thus far Hewitt and Roddick are better in both (playing level wise), but I think in terms of gaps it is already at a point it is much smaller anywhere else than the clay gap is. Australia he hasn't even played at since becoming a contender starting last spring.
Regarding Becker, yeah I agree Nadal is better on hard courts, I was just saying some would probably say Becker is better on any non clay surface (even though I would strongly disagree). Or for a better example many more would say Connors is better on every non clay surface, and in that case I in fact might agree. Nadal's absolute peak might be better on grass, hard to compare when Connors played on real grass and Nadal played on rye grass that Connors probably would have loved, but taking into account their whole careers Connors is possibly better on grass too. McEnroe is another you could argue is better than Nadal on every non clay surface, and probably an even stronger one as more would agree Mcenroe > Nadal on hard courts than Connors > Nadal on grass. And of course Sampras is someone who is clearly considered superior to Nadal on every non clay surface, but would be impossible to rate him higher at this point.