Basement floor slab poured, now frost days.

Pictures and the weather report are sufficient for this. The slab is already poured anyway, so the damage is already done. If there are serious doubts about the proper execution, one must consult an expert.

Informing the general contractor that one is surprised about the execution is a waste of time. And it doesn’t help in court either... in court it counts whether the work is proper or not. And that is assessed by an expert.

If he assumed that it was problematic, he wouldn't do the work.

This presupposes that he is 1. competent enough to recognize the error and 2. assumes that the customer notices the problems.

Here in the forum there are enough examples that can only be explained by incompetence combined with "no one will notice"

What good does that do? A general contractor will hardly say, "Oh yes, good that you told us that. We didn’t know that you’re better off not laying a slab in frost. By the way, while we have you here. We had one of those [Noppenbahn], should we have laid it under the concrete? And what was all that iron for? We took it back, we’re going to build ourselves a grill with it." If the craftsman weren’t convinced that he was doing his job properly, he wouldn’t do it... and he definitely won’t admit that the work wasn’t done properly... Do you think I’d tell a customer, "Well, I have absolutely no idea about this, but we’ll just start and maybe it’ll work out."

With appropriate additives, it is also possible to pour concrete reliably at subzero temperatures ([Zementzuschlag], [Frostschutzmittel], etc.), but of course, this does not come for free.

Concrete C25/30 (standard for non-WU floor slabs) has a cement content of at least 280 kg/m3. At a thickness of 25 cm, the core reaches temperatures of ~15 degrees above ambient temperature. The core temperature is reached after about 1-2 days and then decreases by about 5-10 degrees daily. Concrete is considered frost-resistant if it is maintained above a temperature of 10 degrees for three days.

The foil is sufficient to retain heat and moisture, the floor slab generates enough internal heat, in my opinion there is no reason to worry. For the concrete, these temperatures are much better than summer heat. Lift the foil at the edge and you will notice that it is damp underneath and no ice has formed.