Screed necessary? Unfinished but insulated attic

That should be about right. The screed is denser than concrete. It does not primarily "insulate," that's not its job, it is not designed for that. It doesn’t make that much of a difference. It certainly couldn't fix the fact that insulation was installed in the wrong place here – better than nothing, but the ceiling would have been more appropriate. The screed "changes" this by such a microscopically small amount that it won't justify the difference. The mistake was made elsewhere and cannot be corrected by this (even mitigating it would be so minimal as to be pointless).

Why do you think this construction method is a mistake??
Isn't it more sensible to insulate the building envelope instead of an intermediate floor and thereby let the enclosed space get cold?
The floor between the ground floor and the upper floor is also not insulated, but the entire building envelope (walls & roof of the house) is.

The concrete slab is of course 20 cm thick, sorry for the mistake.

My idea was: Instead of insulating the top floor ceiling (in the attic) and then retrofitting the roof slopes, the roof slope is professionally insulated right away and that’s that.
I hoped to save on insulation that way.

In the end, you also heat the attic, but that is IMHO irrelevant. I would rather ask how the ventilation concept looks up there and whether a (small) heater is planned.

Der Boden zwischen EG und OG ist auch nicht gedämmt,

Do you have underfloor heating?

The attic is to become an (insulated) storage room, it is not to be living space. That is why I will not install heating there.

Yes, underfloor heating on the ground floor and upper floor.

Why do you think this construction method is a mistake?? Isn't it more sensible to insulate the outer shell instead of an intermediate floor and thus let the enclosed space become cold?

Exactly that is the mistake (not in the sense of entirely wrong, but in the sense of differently sensible): the insulation should not be enclosed by the surroundings, but separate warm from cold. For a residential roof, insulating the outer shell would be favorable; for a storage room roof, the uppermost ceiling is the more sensible boundary.

The floor between the ground floor and upper floor is also not insulated,

According to the above explanation, one would only sensibly do that if the upper floor remained uninhabited for a long time.

My thought was: Instead of insulating the uppermost ceiling (in the attic) and then insulating the roof slopes afterwards, the roof slope is insulated professionally right away and that's that. I hoped to save some insulation that way.

You haven't saved anything there: insulating the roof covering instead of the uppermost ceiling costs, for example, practically 30% more insulation area at a 40° roof pitch. Only in the case of permanent residential use of the attic would that have been economical thinking.

In my opinion, a heater belongs there, or the insulation on the floor ceiling. Also, there should be a window or a [Kontrollierte-Wohnraumlüftung].

So normal and sensible are two variants:

1.: Insulate the topmost ceiling and do not insulate the other slanted areas, as they are not heated.

2.: Attic heated, then insulate screed and roof slopes.

This way you inefficiently heat the attic with a higher flow temperature.
In my opinion, this does not make sense.