I believe that's correct: it supports HP "ePrint" branded printers directly and, maybe, any other printer attached to a Mac or PC with iTunes installed by going "through" the Mac or PC.

It's because they don't want to start having user-installable device drivers on iOS (a laudable goal); I assume HP ePrint printers work because they speak something very PDF-like natively, though that's purely conjecture on my part.