So it brokered a protection deal in exchange for nuclear weapons and then... reneged on its promise?
Assuming we sever the last "or" such that the clause activates if "Ukraine should become a victim of an act of aggression," the commitment was to seek UNSC action to provide assistance. America sought that. And it's directly providing assistance. America never guaranteed the security of Ukraine.
[1] https://treaties.un.org/doc/Publication/UNTS/Volume%203007/P...
More would be better, but I don't think we are in the realms of "reneging on promises".
The original agreement was very vague. Was it to mean that the US would defend Ukraine will all it's military might, up to conscripting Americans and sending out nukes? Surely not.