They believed it because a lot of them remember when Russia and (Russophone) Ukraine were one and the same country; saw that Ukraine's government was just as crooked as theirs (but considerably more cash-strapped); and had a continuing suspicion that Ukrainian independence was as much a Western-imposed project as it was an actual grass-roots sentiment.
The cultural and linguistic blinders are real, man. I fucking spent the first seven years of my life in Kiev and the only Ukranian I ever remember hearing was from the occasional street vendor or school cafeteria worker. I honestly had no idea these people were there in any kinds of numbers, let alone that they thought of themselves as a distinct nation.
Now imagine living your whole childhood and most of your adult life in the imperial capital where Ukraine was just a borderland province, one of many, that obviously aspires to be a part of the slavic brotherhood. Of course it's unsuprising they thought the Ukrainians would fold like paper.
That was the only plausible avenue for success. Ukraine is clearly too big for the Russians to swallow. Unfortunately Russia is also too big for the Ukrainians to fight off.