You have to put everything in context of events at the time. The big concern then was that nuclear weapons and materials in the new Ukraine would be outside of treaties, monitoring, and adequate safety controls of the day. Rational people looked at Russia as still being a rational actor and able to absorb those weapons and be accountable for them. Russia already still had several thousand themselves so what’s a few more, as opposed to another independent nuclear power with no track record, and lots of bad actors like Arabs and Norks looking for access to nukes.
The concept of MAD only works when actors are rational. Putin has exploited weakness with threats he isn’t afraid to use them for other than national deterrence.
What is more rational than considering the existential defense of one’s nation? Right now, Ukraine is in an existential war. If they had kept only say... 10 of those nukes... they’d be exchanging mere unpleasantries and nasty words.