From what I understand, it primarily stems from that first stipulation, specifically from points 1 and 4 of the Helsinki Accords
(1) Sovereign equality, respect for the rights inherent in sovereignty (4) Territorial integrity of states
That said, it was very clearly done in a way that didn't actually guarantee that protection, and assuming that the Ukrainians thought otherwise is frankly an insult to their intelligence.
You say that as if the DNCs campaign strategy wasn't to absolutely abandon the left and try to court "moderate republicans", which we very obviously just learned don't actually exist.