The trouble with the substance–production debate (HT Levi Bryant) is that each side in the debate holds asymmetrical of what is being discussed.
For process relationists, production is different from substance. Substances are agglutinations, vortices, reifications, abstractions etc. of an ongoing production process.
For object-oriented ontologists, production is a substance. (Luckily the English term is a gerundive so you can sort of see what this means when you think of production as a noun.)
This is what “substance is anterior to production” means for OOO.
What it means for process relationism is that in the beginning, at time 0, there were solid lumps. Process relationism can't accept this. Of course it can't.
But what OOO means by anterior is that substance subtends production. What does this mean? Production is substance.
Think of a musical: it's a production. It has a producer. The production is a substance. The producer is a substance.
Production doesn't have to mean “process of production.” That already weights the scales in the process relational direction. We've also come to accept the Marxist notion that products are reifications of a deeper production process.
Seen this way, processes are simply objects that inhabit a higher dimensional phase space. When you look at them from the vantage point of the higher dimensional space, they are quite solid and static. They appear to flow in our dimension. So to some extent the notion that mysterious processes subtend visible subsances is a form of correlationism.