what do you mean hard to believe? it worked in the Donbas region, Harkiv is majority Russian speaking... from Putin's office looking at a map it was the same situation, and hyping up the Nazi, ultra-nationalist, anti-Russian threat also seemed like a trivial thing, after all it seems to work in Moscow.
it was a cognitive bias, thinking too much about how and why it could/should work instead of looking at why it couldn't (eg. because the strong selection effect of the repressed political activities of Moscow versus Ukraine, plus the strong rose tinted glasses he and many Russians have that color everything positive that seems like the old Russian Empire, whereas Slavic people a thousands of kilometers away have a rather different view of how great that is and was.)
It didn't work in Donbas in 2014 either. Neither it did in Odesa and other cities during Maidan. The level of domestic support was unexpectedly low (for them), that's why they had to rely on local criminal gangs and resort to recruiting the far-right activists from Russia by May 2014 (such as "neo-Cossacks" or former RNU members), then the military vets by June. The native separatist movement in Donetsk that existed there in small numbers has been pushed aside.
Russians are a lot of people. some wholeheartedly believe that the current war is started against the Nazis who hold Kiev and hunt the Russian population.
and Putin likely counted on at least some support from some parts of the population in Ukraine. as evidenced by their plan of quick victory, and that the Ukrainian government will collapse, and it will be easy to install a puppet government. just as Janukovic was relatively popular some years ago
