The same article says the US itself claimed the Memorandum was not legally binding when it sanctioned Belarus. And the Analysis section starts with a clear:

The Budapest Memorandum is not a treaty, and it does not confer any new legal obligations for signatory states.

It also states that many Ukrainians at the time considered that keeping the nukes was an unrealistic option since all maintenance and equipment required to maintain them were located in Russia, Ukraine was under a financial crisis at the time and had no means to develop those things itself. I just can’t understand people now claiming it was a mistake to give up the nukes. Russia might have reasonably invaded Ukraine as soon as it was clear they intended to keep them as they knew they didn’t really have the ability to use them and no Western government would support them using them and starting a war that would likely contaminate half of Europe and cause terrible loss of life. It was absolutely the right thing to do for Ukraine. Even if that didn’t save them from future aggression, which I think was mostly the fault of the West for not being prepared to really sign a binding document and put the lives of their own soldiers on the line.