The full story is that the modder for the 3PG community (Pawn) had been working on this since the day Portal 2 was announced and didn't intend on letting anybody know until it was much further along. However, valve put a policy in place last month that bans custom items from being given to players, in an effort to stop servers from giving people valve made hats. He was ready to scrap the whole thing until we found a temporary hack to bypass the ban and now he just wants to show off what he has done while he still can.
I have heard rumors that valve has as of last night started reversing this policy, so now we can get back to work polishing this thing up for a full public release. From the way Pawn talks I assume the full version will be exclusive for a month or so then released publicly.
A few things that I don't believe are mentioned in the video:
-The portals can be destroyed by shooting them, so if the enemy is doing something like setting up a portal between the two flag rooms you can just shoot the portals and shatter them. The current HP is about that of a sentry but it can changed to make it fair.
-The gun physically replaces the secondary weapon for the classes that get them. The medic and spy don't get them as the game treats the medigun and the diguise kit as their respective secondaries.
-The third person view in the video is just for demonstration to show that it is indeed in TF2.
-It is apparently possible to add textures that are immune to portals in the map editor, so a map made specifically for the mod would hopefully be grief proof.
-The intention is to make this a whole new game mode with its own unique maps designed around the new gameplay.
-At this point the early alpha version shown in the video is only available on our secret internal test server. I would rather open the test server up to the public, but the others prefer not to let most people use it until it is further along.