Definition of topsoil is unclear to me

with our builder, it is written in the contract for work that it will be filled with 15 cm of topsoil,

So it probably does not say that in the contract for work. The exact wording would be important. And then whether it is a builder from whom you buy a house with finished exterior or a general contractor who puts your existing soil back where it belongs.

do I have to accept it as it is shown in the picture here

see above.

According to various sites, it is always the top layer

Yes, grown existing soil. The photo seems to show quite normal earth substrate/topsoil, which is ideal for planting, e.g. for lawn. Real compost soil would be much too rich; you add that to demanding plants and it is not topsoil.