It would be true if Karlovic and Isner didn't exist. But they exist and can make such serves, which are impossible for all the rest, i mean about angles ..You know, I've tried to do a bit of math on this ('cause I'm the geeky type), and what I come up with is that a 6 inch height difference doesn't really translate to very much, angle-wise, over a distance of 60+ feet. A serve down the T, landing on the service line, (neglecting spin, gravity, curving and other non-Pythagorean elements, but considering average arm vs height length, racquet size and jump height), is gonna have an angle of 9.9° for a 5'4" player and 10.9° for a 6'2" player... really, that's not a lot. The same goes for a serve that just makes the net; there really isn't a huge (mathematical) difference.
That said, I don't doubt that height makes a difference, but it's not just simply about angles.