Do you have evidence of Marx and Engels, both who railed against so-called "anti-authoritarians," saying that "Democracy must be abolished?" Do you have evidence of Marxists who followed them saying "Democracy must be abolished" either? I do not believe you will.
What I do see is Communists advocating for the destruction of Capitalism and the structures that support it, replacing them with proletarian democracy.
It isn't a non-sequitor, your point itself was a strawman that doesn't exist.