I highly contest the first part of that statement. Marx and Engels were not particularly keen on describing and outlining what 'Communism' would look like, with that vagary being a major contesting point between multiple communist and leftist factions. They did believe that it has to come, or it would bring about the ruin of the contenting classes, but what it would look like and how it was organized was deliberately vague because it would be like someone in the feudal system trying to describe capitalism. What was clear to them was that money and the state would need to be abolished, as they would be unnecessary to a freed working class, but how those relationships would be established was not described.

That said, there is no better explanation and description of capitalism than Marx and Engels, to have any real understanding or build an even remotely close model of modern political economic realities, you have to start with them. That doesn't mean you can't get into some weird offshoots, but JP's problem is that he hasn't even read any Marx and Engels in the first place. He can't even describe what they are actually talking about, let alone present an adequate critique of it (which there absolutely is).