If this is true they would’ve still had material to go on. Unless the unpublished books are as illogical as the last few episodes of the series has sadly become. So I doubt there’s a fully written final two books on the shelf in GRRM’s bedroom. However I would believe that there do already exist outlines that could be more or less detailed. For example I think it was said that the ‘Hold the door’ moment plays out similar in the unpublished books. I might remember that wrong though but as an example.