It was not even an agreement at all, nevermind binding. It was words said by the Russian president. Not a contract, not a memorandum, just words in a meeting. If Ukraine took one sentence said by the Russian president in a meeting as security guarantees from US (emphasis on the word guarantees) then they are the problem.

Security guarantees would at minimum require a treaty ratified by congress in US, something which I don't think is on the table any more at all.