They did no such thing: https://politics.stackexchange.com/questions/7899/why-dont-s...

The leaders just signed a piece of paper which was never ratified by legislators and doesn't even guarantee what you claim:

> The United States of America, the Russian Federation, and the United Kingdom of Great Britain and Northern Ireland, reaffirm their commitment to seek immediate United Nations Security Council action to provide assistance to Ukraine, as a non-nuclear-weapon State Party to the Treaty on the Non-Proliferation of Nuclear Weapons, if Ukraine should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used.

It was just a promise to have a meeting at the UNSC if something happens.