Does frost get under a slab on grade?
I bet you will find frost under your FPSF in northern Minnesota end of February if you did..
I think your slabs manage frost in hard freeze areas, not prevents it which is ok unless you have unequal heave.
I want to eliminate ALL concrete, by replacing soil with washed gravel 4' deep under my permacolumn Pole barn house walls. Rubble foundation. I would use FPSF insulation methods with rockwool, but build wooden floors over drained gravel, rockwool, and vapor barrier. But I'm reluctant, because I think the chance of heave is still high and I won't have a rebar slab to ride it out up here in Iowa. The site sits on top of a hill with great drainage, is why I'm considering this method.