It's probably more accurate to say that the loan you took and paid back was insufficient to establish a high credit rating due to insufficient history, rather than "wrecking" existing good credit.
Right, I also paid off my loans within two years, and it doesn't seem to have affected my (very good) credit rating negatively. I've also had a credit card in my name (with my dad as guarantor, at first) since I was about 16 -- and he has sterling credit. That helps a lot.