Legality isn't that much of an issue during a revolution, and what happened in the Soviet Union in 1991 was pretty close to one.
As far as the Union Treaty goes, Gorbachev was about to sign a new one he'd negotiated with the Republics in August 1991. Then the shit hit the fan. Here's a pretty good detailed timeline of the events of that period:
http://www.bbc.co.uk/history/war/coldwar/soviet_stand_01.shtmlIt seems like the ultimate disbandment of the Union Treaty was the various Republics declaring Independence from it. I couldn't find the actual text of the treaty anywhere, so I'm not sure if that was 'allowed' in the treaty or not.
It's a pretty thin argument for the continuation of the Cold War, in any case. If that's the worldview they want to use, they're probably WAY confused by the multipolar world we're in now, our own self-image notwithstanding...