Yes, I think it’s pretty clear that for all his pretensions of expertise Juvenissum really didn’t understand what he was talking about.
What I think he misunderstood is this. Granite is formed when magma slowly cools. If that magma is hydrated, the water is released. That’s the reaction he was describing. If the magma is not hydrated granite can still form, but you won’t get an6 water out of it.