Not meaning to start trouble, but I've heard variants of this claim over the years ad nauseum. Personally, I don't believe it as it makes no scientific sense whatsoever. Why would only the alcohol be absorbed? And where is it absorbed to? Fruit may indeed by very porous (especially melon), but it's like a saturated sponge - there's no room for more liquid to be absorbed. If you dip a sponge or cloth into the punch, the entire mix of liquids will be absorbed - the alcohol won't magically separate from the punch and be soaked up. Nope. Not buyin' it.
I call BS on this one. Someone prove me wrong - please.