Not a silly question. And the answer is in two parts: I did not click on Internalise, but on opening the GH file in a new model I find that it is. Given that all I did was to set the surface, add the dimension component and resize in Rhino to check the dimension response, I can be 99% certain this wasn’t just a “senior moment”, so how it got to be internalised is a mystery.
There was some faffing with the object selection for the set which may have something to do with it, but I doubt I can reproduce it, so I think this will have to be chalked up to experience and left behind.