SE requested to use Gen1 as blinding layer over the hardcore. The builders say that to fill the gaps it will be a runny mix and it will take weeks to dry before rigid insulation can be put on top.

I argued that concrete sets, not dries, but they may be right in this case, as with excess water that cannot be bound chemically, it needs to evaporate.

Is there any admix to speed up floor readiness, or is dehumidifying my only solution?

You can use a cement substitute such as Ardex A38. We use that in screeds to get on them in as little as 4 hours. There are others out there. 
I’ll message my screed guy and see what he recommends. 

There’s a big difference between hardening and drying out 

Your builder is being over cautious 

 

 

 

You should have a DPM so no issue with it drying.