Flat duct in screed

Hello community,
we have decided on a slot ventilation in the kitchen and need a flat duct for it.
It has the following dimensions 220/90mm with a length of 1800mm (1000mm under the kitchen island and 800mm under tiles) and is to be mounted on the raw floor.
The subsequent floor structure is 15 cm including tiles. (underfloor heating)
The architect has now told me that this would only be possible if the height of the duct is 55mm instead of 90mm.
Is this statement correct and do we have to accept it?
Regards and thanks

We have installed a future flat duct for the fireplace and according to the screed installer, 60cm height was also the maximum limit for a floor buildup of 18cm (without parquet!). The flat duct may only be placed in the insulation and must still be able to be insulated on top. But you can simply take a wider one? That should also exist?

why is it necessary to insulate the duct?
Is this primarily about decoupling in terms of noise levels or to avoid thermal bridges?
Regards

Yes, because of thermal bridges. Although for us it was the future for the chimney. Don't know how it is with the exhaust air.

But it should be sufficient if the channel is only insulated from below?

I have no idea. I only know that at our place, the water pipes were also insulated at one point. This was specified to us by the chimney builder and the screed layer.