In the mixed economies that have existed to date, the socialist elements have remained subordinated to the capitalist elements.

There are no 'socialist elements' in a mixed economy. It's a capitalist economy because the mode of production remains capitalist and the commodity form exists, it's only the market that takes a simpler form (monopolized) with a single element on the 'job supply' side - the state.

Nationalised steel industries within a capitalist system have to conform to market pressures. Similarly the “worker receiving full value of their labour” in worker coops operating in markets will also engage in commodity production. Key is that with a planned economy, anarchy in production has been eliminated and the scope of commodity exchange has been reduced.

I fully agree with this point, I just don't see the relevance. It's still key who plans the economy and manages surplus value, right?