Is a laptop mobile? Is an iPad, Slate or Android tablet mobile? What if it has a (bluetooth) keyboad? Is it still mobile then? Is using an iPhone YouTube client but throwing it onto the TV via an Apple TV box mobile? Ditto Android Phone to Chromecast dongle? Is that mobile?
The point here is that mobile and non-mobile are rather blurred and irrelevant terms.
What Google are celebrating is not that they have 40% of traffic from mobile, but rather that they have sucecssfully maintained the YouTube monopoly of video content supply, despite the move to mobile platforms.