It's not your property. The landlord can have it done like that. It's clear to all of us that it's not sensible. It's also clear that it would be better the other way around. And that he won't give in because of such a fuss is even more stupid. But not everyone is good at admitting mistakes. It just really turned out badly. But legally, there's no basis for that.
No, that's not what this is about in my opinion. The landlord has certainly recognized the mistake, but why should he correct it at his own expense, i.e., pay money?
The tenant is already about to correct this problem on his own? And if not, the tenant has to live with it, not the landlord. The tenant has apparently made it clear that he will take the apartment even with this flaw...
Important info: Value increases of the apartment, financed by the tenant, may be offset by the landlord with a rent increase.