This is not correct. In order to get Kensei II, for example, you will have to have purchased X amount in the Kensei tree AND be level 12 fighter.
Quote from Eladrin via PM: "A 2 level splash could replace all three of their trees with those from the splash class, but due to level restrictions on enhancements they'll probably be restricted to the bottom two rows of enhancements, and won't get the free ones. (Those require levels 3/6/9/12/15/18/20.)"
Bottom line, aside from SOME feat reqs. There is still a min class level req in addition to the AP spent req. for PrEs