Cheesesnipe, it's kind of how it works, but not exactly. When you add spores to milk, those spores will rehydrate and will try to grow and release a very small amount of enzymes inside of the cheese. But they only contributes very slightly to ripening because they never have a chance to grow-- no oxygen inside. They do affect the rate of maturation, like you said, but it's very minor. The ripening is still almost completely from the outside in. If your paste was runny, the adding-to-milk is likely a red herring. Runny paste is more likely caused by too much moisture, too low pH at draining, not enough fat in the milk, ripening temp too high, etc.