ROL licensed *RISC OS* from *Pace*. Castle bought Pace's head license. Therefore whatever obligations ROL had to Pace - ROL now have to Castle.
It is *my* understanding from what was published at the time that you are correct that Castle can't (as a condition of purchasing the head license from Pace) GPL the RO5 source - however as *much* of Select is *still* just plain old RISC OS - yes the one Castle own the license for - then the same obligation extends to ROL. That's precisely why I suspect guestx suggested that people buy out Castle's rather than ROL's license as Castle's license is the *main* one.
As to Select 4 yes any parts of it that don't exist in RISC OS are indeed ROL's intellectual property - but given that these run on good old RISC OS (which ROL still have to license) the license terms still exist. I'd point to ROL's partial open sourcing of the Printing system - but they were unable to release *all* of the Printing source for that very reason. They could only release the bit's they made - not the bits that already existed and which they need to license off Pace (now Castle).