The first reunifucation document cites article 23, indicating that the GDR would enter as states (Laender) adhering to the FRG constitution, which was subject to article 29. The only other option for reunification provided for in the FRG constitution was a negotiated rewrite of the constitution, itself requiring a plebiscite, which they did not do. The “contract” does not mention article 29, but it is subject to the only two provisions in the FRG constitution for the reunification (accession if states via plebiscite and negotiated rewrite of the constitution, also requiring a plebiscite).
After the fact, liberals began coming up with explanations for why this blatantly illegal “contract” was actually fine, including things like the sourced document conflating general elections of parties with a plebiscite.
I’m saying the FRG broke its own laws to annex the DDR. I haven’t said that this is inherently “the wrong thing”, I note it being illegal because liberals pretend to care about such things.
There is no provision for this in the FRG’s provisional constitution at the time. I linked to it and explained the two options already.