It would make thematic and logical sense that a flying model is exempt from disengaging strikes while passing overhead, a floating model is not. Seems a floating model merely hovers (maybe a few feet) off the ground, allowing it to ignore terrain, otherwise forcing it to deal with nearby enemies wanting to take a swipe. Of course, flight and float would be a non-issue when attempting to leave melee range from ground level, which would trigger a disengage strike as per normal.
I would think flying models would be exempt from any effect or trigger that require it to be within melee range or a specific radius, at least while passing overhead and not assumed to have landed (between activations, for instance).
There's no source to cite, it just seems right. In the absence of an official ruling, I tend to err on the side of logic (with my fellow players agreement, of course).