Window reveal before window installation... smooth coat?

So this is absolute nonsense and has absolutely nothing to do with DIN-compliant preparation for window installation. As you already told the OP, a smooth leveling coat must be applied to ensure that the compressible sealing tape and the adhesive tape seal properly for airtightness.

So the window installer says... it's OK. Still, as a layman, I’m worried^^

So I have consulted with the shell builder, site manager, and window installer. All three say that it is okay... Well then... three opinions on site, what am I supposed to argue against now.

Then definitely take plenty of photos during the installation and, if necessary, post them here again.

Did you mention the smoothing screed?

So I have consulted with the shell builder, site manager, and window installer. All three say that it is okay... Well then... three opinions on site, what should I argue against now.

Oh dear, I know that from our construction, all parties also said it was okay. In the end it was crap, and none of them were reachable or interested anymore. It should not be like that.

This morning I consciously paid attention, at the three shell constructions in our immediate neighborhood a smooth finish was done everywhere.