Of course it's just a dishonest pretext by Putin, he has no problem with neo-nazis as long as they're on his side, but there's a grain of truth in it. Look up Azov battalion.
Of course it's a rotten excuse for an invasion. But I think it answer OP's question, "why on earth anyone is buying into it". People are buying into it because there's a grain of truth to it.
Who BTW are funded and equipped with our tax dollars. Yay!