The only thing mandalorian about Fett was the armour. It's been that way since before the prequels.
Just read the bio on the back of Hasbro's Power of the Force Fett figure or the Visual Dictionary. They both clearly state that he wears mandalorian armour but nothing about Fett himself being mandalorian. In fact, the bio from Hasbro even stated that the mandalorians were destroyed during the clone wars.