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Jarate dates back over a year ago, from the sniper movie. when hes camping out hes filling the jars up.

That's not jarate. That's just the sniper's way of relieving himself without giving away his position. Jarate dates back to an April Fool's Day joke. And if it really was fitting with TF2's humor, it wouldn't have worked as an April Fool's Day joke at all.

Thats exactly what Jarate is...
Mann Co. sells the jars, you are responsible for filling them up.

Right from the horses mouth:

Jarate, the martial art of Team Fortress 2 involving thrown jars of urine, also was birthed outside the game, a little through the Sniper's movie and a Valve comic strip. Walker said that its introduction to fans through set up a perfectly understandable context for his game design team to introduce a gameplay element they desired, a "short-range enemy de-buff." No need to invent something new for that. Jarate was already in the fiction and ready to be implemented. Walker sees this as one creative end of Valve helping build the work of the other. "It's the natural result of having other creative folks messing around in the same sandbox. It's really hard not to see what they come up with and want to use it."

http://kotaku.com/5273555/valve-dreams-of-team-fortress-2-movie-divulges-meet-the-team-origins

The point is, when that video came out, there was nothing to suggest that the jars were meant to be thrown at your enemies. No more than the introduction of the sniper's parents suggested that they'll be playable characters in the future. No more than the Meet the Soldier video suggests that the Soldier Update will include the ability to rip off your enemies' heads and stick them on a fence. It wasn't Jarate then. It wasn't even foreshadowing Jarate. It was just a brief gag without any significance, and a future April Fool's Day joke would be a reference to that gag.

And if Robin Walker thinks that an April Fool's joke counts as canon "fiction" for the game, he's doing it wrong.