Well, I certainly wasn't making the argument that katana are magically awesome weapons or anything :P I don't think anyone here would make that argument. The only argument I'm making is that Jomon with nerfed infantry seems like an unnecessary nerf, especially since they weren't that amazing to begin with. Vanilla jomon for example is pretty bad at expanding because they don't have any survivable units they can recruit, and everything that they can recruit takes a lot of resources even so. So they lose tons of units expanding and have to replace them.
As far as the video goes I'm not sure it's entirely accurate. In any case I've just seen some videos of katana cutting through steel pipes, sheets of metal, and japanese steel helmets well enough, along with various other objects. So while it may not be a wonder weapon, it also isn't an atrocious one. If I were to stat a Katana in dominions I would probably put them at 6/2/2/3 (vs a broadsword at 6/0/1/2), which is one less attack and one more length as compared to how illwinter did it. Mainly because a two-handed katana is a much faster of a weapon than a 1-handed broadsword is. I'm not sure I disagree with 3 attack though. If you compare the two and consider the samurai isn't using a shield, I think that's pretty fair really...
Counter-link
http://www.youtube.com/watch?v=EDkoj932YFo