This is total nonsense.

I’ve heard your opinion. Still it’s a fact.

their declaration of independence was entirely unilateral and decided on not by referendum

You may be mixing up the declarations of sovereignty of the union republics and their actual secession. The former happened earlier than the latter (though for Kazakhstan the latter never formally happened), that is, happened without leaving the USSR, and didn’t require any referendum.

I’ve just read that Estonia, in fact, had a referendum in accordance with the Soviet law.

So did Armenia, Georgia and Azerbaijan. I can’t doubt its results were correct for Armenia, since Soviet troops were in fact committing ethnic cleansing in Artsakh even as that referendum was taking place (which was the main reason for Armenia leaving USSR as well). For others - likely correct, but I don’t know whether real or not.