I'm not sure if they did either, it sounds plausible but difficult with the lack of root browsing.
Anyway if you go to the test tube section of YouTube and try the HTML5 option it comes up as the Wii U using flash just like my computer did with the Wii U's user agent. It's an easy fix for them to just specify in the code it's an HTML5 only browser especially since that's what it was detecting prior to the issue.
My only concern is how to get YouTube's attention and if we already have it a statment to specify why other devices received a fix but the Wii U hasn't. This isn't a small company and this fix, considering they're not purposely doing it, can and has been fixed within a couple hours. So to any YouTube devs that address and oversee this section of reports: the issue has been identified, what course of action can we expect? Thank You.