The source of this claim is a tiny anonymous telegram channel, that was then cited by NY Post, which was then presumably quoted by Ratcliffe.

The Budapest Memorandum was initially signed by Ukarine, Russia, United States and United Kingdom, “assuring” the security of Ukraine in exchange for giving up their nuclear arsenal. But afaik it doesn’t really imply any legal obligations. I assume that the new agreement would involve more concrete conditions and responsibilities.