I think josephlord's point was that the name "ROT13" completely specifies the 'key': If you changed that, it would no longer be ROT13. So it arguably can't be considered a key if you define 'key' to mean 'a piece of information additional to knowing the algorithm used that is necessary to decrypt'. (If the 'algorithm used' was specified just as ROT (i.e. caesar cypher) then 13 would be a key). But this is basically semantics.