a slab in designed accordingly. The difference between 85 and 100 is well 15mm. I wouldn’t worry too much. More important to go with a builder you like. But in saying that i do like there tactic. Probably only cost them an extra $400 in concrete and it is something that no one else would match, because they know they don’t need too.
It is very common for waffle slabs to be done with 85mm concrete here in west melb. Where as conventional is normally done with 100mm top as the trenches are much further apart.