So what does "in earnest" mean to you in this context? I understand it to mean "fa realz", just to keep things simple.
There's a very big difference between something beginning, and something begining in earnest. WWII began in 1918 with the signing of the treaty of Brest-Litovsk, but it didn't beging in earnest until the German invasion of Poland.
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.