The trick with what TCNA/ANSI states is in the word plane. There are out of level floors and out of Plane floors. you can indeed tile an out of level floor as long as the space to be tiled is in plane .Of course the better bet is to first level the floor properly (deflection)?

an 1/8-1/2" out of level I usually correct by floating my tile install or wet setting. it all depends...working within the peramaters of the medium bed mortar thickness specs if i can do it that way and if it will save me or the client the time /$ of leveling and still achieve industry standards...........