Baker's un-written assurances may have bound him (Baker) to personally help ensure his assurances held, if only to preserve his credibility in other negotiations, but all parties involved understood they were not being written into the agreement and therefore were not a formal agreement. These words had meaning, even when not written into the agreement, because Baker was telling Gorbachev the truth, which was that the US understood that moving the borders of NATO east would be a bad idea. Gorbachev heard that the US understood this. It wasn't a binding legal agreement, it was a statement about the political landscape at that time.
Baker is long gone from the diplomatic landscape and so is that assurance. This isn't actually a surprise.