The object complies with the KfW standard. What does that mean?

At the offering company. There is no "KFW Standard," the reliable term is "KfW Efficiency House 55/40/40+."

Do you already have a contract? Or are you still in the [Angebotsphase]?

Have already signed the contract. ..

Then the construction method, insulation, etc. are described in more detail anyway. Careful reading of what has been agreed upon helps; you won't get more than that.

Where can I get a legally binding answer cheaply?

From your lawyer. Nobody else is allowed to, otherwise ^^

So you want to enforce kfw55 for free because of the sentence

"The object complies with the kfw standard"

Even if you might basically be right, the wording is just very vague (as is so often done in the construction specifications...) and basically a marketing gimmick. Legally, they will find a way to wiggle out by saying "the statement is unfortunately outdated from the old energy saving ordinance or something similar." And as already said, Kfw 70 is the standard today anyway. So, basically, their statement is somewhat correct after all.
Don’t make a big deal out of it and that’s that.

PS: Normally, ALL open points of such an important description are clarified before signing. Just saying...

KfW Standard is the Energy Saving Ordinance 2016, the former Kfw70... no upgrade to kfw55, not worse than Kfw70, so the former 100.

Energy Saving Ordinance 2016 is the current standard!