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.
I like how one of their points of evidence is a only kinda sort of expensive camper trailer. Yeah, I’m perfectly fine with the people maintaining a public resource like that being able to go on camping trips lmao.