Yes, I don't really understand the linked blogpost. I mean, if it says 110 V in and 15 V out, that is probably what it does, regardless of whether they are using some parts with "220 V" printed on them. Plugging that into 220 V would probably not put out 15 V. And I agree that putting in 110 V into a European adapter would probably result in a lower out voltage than 15 V. Did he even measure the voltage at the output end when connecting it to 220 V? I don't get it.
Yeah, the blog post doesnt really go into detail, but one thing is clear though. If you plug in something hooked to a power supply designed for 110 volts into a power outlet that gives you 220 volts, you will normally fry whatever equipment is connected to it. And apparently, hooking US Wii U's up in Europe works.
Ill go and get a converter just to be on the safe side