The beginning of that quote is worth adding for context for folks unfamiliar with Engel's argument here:

Why do the anti-authoritarians not confine themselves to crying out against political authority, the state? All Socialists are agreed that the political state, and with it political authority, will disappear as a result of the coming social revolution, that is, that public functions will lose their political character and will be transformed into the simple administrative functions of watching over the true interests of society. But the anti-authoritarians demand that the political state be abolished at one stroke, even before the social conditions that gave birth to it have been destroyed. They demand that the first act of the social revolution shall be the abolition of authority. Have these gentlemen ever seen a revolution?

And his conclusion:

Therefore, either one of two things: either the anti-authoritarians don't know what they're talking about, in which case they are creating nothing but confusion; or they do know, and in that case they are betraying the movement of the proletariat. In either case they serve the reaction.

The short entire essay is worth reading for other folks reading.