Of course, they need a lot of documents for the finished financing contract that you don't have yet. We have now received all documents from the notary 4 weeks before the notary appointment (also to review the purchase contract [two weeks prior are legally required]).
If I know that you are getting a loan from the banks in the amount of the allkauf house price, then it's no problem to sign at Allkauf Haus that you can arrange financing within 3 months. Allkauf Haus sends you all the necessary documents and in about 2 weeks completes the financing.
If you already have a verbal financing commitment, then the question is rather whether to build with company XYZ. But if you have already decided on Allkauf Haus, then what bothers you about this contract clause. It is not really a surprise that a developer wants to see financial security for the construction project.
EDIT:
Here is the excerpt from our contract:
§ 9 Financing
(1) The procurement of external funds is exclusively the responsibility of the buyer. The due date of the purchase price installments is independent of whether, when, and how the external funds are paid out and made available.
(2) The buyer is obliged to prove to the seller upon request, at the latest upon the due date of the first purchase price installment, the provision of the financing and/or equity required for the total purchase price.