Basically, you can't have a home console generation with only one entrant, so even though it begins with the Wii U, it doesn't begin in earnest until there are at least 2 competitors.
You wouldn't really say that the previous generation began in earnest when just the 360 was out, or that the one before that began in earnest with the dreamcast.
That makes perfect sense. The others wrote something like this before, but this was the best explanation, sounds right. I think "fa reaalz" was a good way to slang it ;) but I understand your subtle distinction now.