To me it's just a limitation based on the world as seen by these models. They know there's a letter called 'r', they even know that some words start with 'r' or have r's in them, and they know what the spelling of some words is. But they've never actually seen one in as their world is made up entirely of tokens. The word 'red' isn't r-e-d but is instead like a pictogram to them. But they know the spelling of strawberry and can identify an 'r' when it's on its own and count those despite not being able to see the r's in the word itself.