Why did the insulation company refuse to put rockwol insulation blanket on top of the polystyrene chips alread?
There are guidelines on how insulation should be applied and I suspect they are following their company policy. I can see that there may be a problem of filling the gaps in that the current space between your rafters is only partially filled and to just lay a rockwool blanket over may not do the job properly. Additionally their may be some issues in mixing the two materials, regardless the company should explain precisely what the problem is.