Sorry, I should have explained better. I'm not saying the Wii is a last gen console. Of course it's part of the current gen. But therein lies the problem with Gabe saying the Wii U is more powerful than the "previous generation." The 360 and PS3 are certainly not yet part of the previous generation. They're a part of the current generation of consoles, as is the wii. They wont become the previous gen until the next gen gets here.
So when Gabe says the Wii U is more powerful than the previou gen, he could only really be talking about the Wii because that's the preceding nintendo console. If he had said current gen, it would be a more inclusive statement that would cover both the wii and the HD twins.