Does your general contractor build the drywall on the screed or first the wall and then the screed? Retroactively, it is only possible on the screed.
I personally find soundproofing through proper execution important. Therefore, the wall should be installed first, and then the rooms should be separately filled with screed. Also, things like underfloor heating, more electrical work, painting, etc. should be considered. You don't want to drill into it yourself right away during your own work.
I would have it explained to me exactly again what he plans to do for the money, the sequence, etc., and then possibly negotiate the price down to an acceptable level.