"...the notion of people paying Castle on a per-use basis shows an element of ignorance that would suggest that Castle merely want to offer the code under some kind of 'shared source' agreement."
Perhaps I'm misunderstanding what you're suggesting here, but I don't see why (as others have suggested) Castle can't be talking about some kind of dual licencing agreement. Lots of open source vendors do this, allowing them to release the source under a genuine open-source licence, whilst at the same time charging commercial users who choose the commercial licence: [link]
Timephoenix makes a good point about the ROMs. There's nothing to stop RO Ltd carrying on in exactly the same way they already are even with RO open sourced. They just use the commercial licence or charge for putting the open source version into ROM.