It technically is.
- Crafting = item forging in RPG's
- Hats = a special armour
- Achievements = quests
And more.
RPG=role playing game. Now, if you want to get technical like in your post, almost EVERY game is a roleplaying game. In Halo, you play the role of some spartan alien thing, therefore, by ARC's logic, it's a role playing game. In super mario games, you play the role of mario. Not a platformer, its an RPG says ARC.
It's an RPG with FPS elements.
Kinda like Borderlands.
Borderlands is as much an FPS as an RPG because you collect items and level up and what not. TF2 is an FPS with SUPER slight undertones of things that happen in an RPG game. Just because a game has unlockable things doesn't turn it into an RPG, and the hats system isn't RPG related at all, just a little add-on that has no value whatsoever. To make TF2 an rpg, Valve would have to add levels and ranks, and the ability to make your health go up, make bullets do more damage.
On a completely diffirent subject:

...now let the wars begin.