Ukraine was "not negotiable" Russian according to multiple statements from Putin, so that part about "negotiating concessions" is provably false.
Why that was the case is indeed complex.
Russia did not try to negotiate Ukraine away initially. The messaging was clear: it's ours. Now they might negotiate something because Ukraine is kicking their teeth in, but that's after the fact and completely forced.
Lies of the Russian government about non-negotiability can be ignored much like the statements about non-invasion turned out to be lies. I don't see how they'd prove anything.
Why that was the case is indeed complex.
Russia did not try to negotiate Ukraine away initially. The messaging was clear: it's ours. Now they might negotiate something because Ukraine is kicking their teeth in, but that's after the fact and completely forced.