Not so absolute.
If a consumer is a contracting party, the content control of general terms and conditions according to the Building Code applies as a corrective and takes precedence. It may "invalidate" clauses that are disadvantageous for consumers.
*Edit:
Simply handing over is not enough for consumers inexperienced in construction.
The contractor must then also comprehensively inform the client about the disadvantages and be able to prove/verify that this explanation took place. In particular, the client must agree to the inclusion of the general terms and conditions (here: VOB/B) (§ 305 II Building Code).
The VOB are furthermore not legal norms/laws, but general terms and conditions, which must be agreed upon as such. The VOB targets persons experienced in construction. Merely handing over the VOB documents does not convey this experience in my opinion. And as mere general terms and conditions, they are subject to content control according to § 307 Building Code.
For further study, copy and search, presented there well and comprehensively:
7th Symposium on Construction Law of the Chamber of Crafts Dresden vob
There, the presentation also addresses at the end the reduction from 5 to 2 years and maintenance.
Since in the case of the TO here the VOB was modified and not agreed upon 1:1, the Building Code applies in my opinion in his favor again, so unanimously 5 years for the building structure.