I solved the problem. I was using psql.exe from pgAdmin III and ran pg_dump.exe from the same folder. the psql & pg_dump binaries included with pgAdmin III are version 9.2.2 (pgAdmin 8.3 I think?, in any case, it's straight from the installer from the website advertised as 9.3.4, I didn't have postgres on my computer before the install)
However, I realized there were also the postgres binaries installed somewhere else, and those are the right version. I also set up my PATH variable to avoid issues, however pgAdmin was still using the old binaries so I copy-pasted from the postgreSQL folder.
tl;dr postgreSQL is fine, pgAdmin is shipped with the wrong version. Not sure if that's supposed to happen or not.