It's tricky to argue labels. To me (right or wrong) platform suggests support, active development, community, backing, a reason to believe that development will continue and something you can build on. Of course, you can call platform whatever you like though.

Ghost has many (all?) of the above, and that's relatively rare in any ecosystem (PHP, Ruby, Python, whatever) for a young project. If this project can continue building momentum, then it would truly be unique in general and certainly in the JS ecosystem.

ETA: When I say tricky throwing around labels, I mean one thing can mean one thing to me and something else to you. I guess semantics would be the short word for that.