Construction according to the Energy Saving Ordinance / GEG, assessment KfW55

Do you consider the statement plausible that, from his perspective, complying with the KfW 55 requirements would have caused disproportionate additional costs?

It would be very frustrating to be told in six months, "Sure, with a better ventilation system that would have cost 10,000 € more, KfW55 would have been possible."

Maybe the house was built/calculated just barely at the limit of the Energy Saving Ordinance KfW55, and he simply does not want to commit to KfW55.

That would at least fit his statements and could certainly be the reason. The question would then be what I could do to get him to guarantee it after all. Could I suggest (for an additional charge) installing a better ventilation system (currently central exhaust with supply air via window frame ventilators), photovoltaic panels on the roof, a more expensive front door, paying for experts myself, or something similar? Or does all of that not make much difference because potentially other more weighty points counteract it.

I don't have all the values either, but after reading various pages and forum threads, I would have thought that regarding the building envelope, a thermally insulated base slab, exterior walls with thicknesses of 36.5 or 42.5 cm with ETICS, and a roof insulation laid with a slope with a maximum thickness of 34.5 cm already provide a good basis. A heat pump is also present. Or put differently: Either I build according to minimum requirements to save costs (and that does not seem to be the case from my layman's perspective) or I build to a better standard and then would not end up just barely above KfW 55. Or I don't feel like dealing with the paperwork for KfW 55, which could perhaps be solved by personal contribution or financial incentives.