Why are Delta Aquariids "for the southern hemisphere" while the Perseids are "for the north"?

I suspect that that is simply a statement about where you would have the best visibility of each meteor shower. The name of each shower tells you where the radiant is, the point in the sky from which the trails of the meteors appear to originate. Perseus in the northern sky (with the specific radiant point of the Perseids at +58 degrees declination), while Aquarius is farther south (with the radiant of the delta Aquariids at about -16 degrees declination), so a northern observer would have the Perseids radiant higher in the sky and would see more of those meteors, and vice versa for a southern observer and the delta Aquariids.

Since the meteors go in all directions from the radiant, at least some from either shower would be visible for observers over a large range of latitudes, but you’d see the most if the radiant is high in your sky.

Edit: this nice image from spaceweather.com shows clearly why the radiants have their respective locations:

(From spaceweather.com, linked from Sky & Telescope.)

These are reconstructed orbits of fireballs seen as part of these showers. The light green are the delta Aquariids, and the darker green / teal are the Perseids. You can see that they have very different inclinations where they intersect Earth, so they appear to originate from different directions in our sky.