Yes it's true, all Wii U's are vulnerable, and Nintendo is even still actively selling entry-points that allow you to access homebrew persistently.
What's an entry point?
The Nintendo DS emulators on the Wii U eShop are vulnerable to an exploit called haxchi.
For now, the site ain't much to look at, so give these links a look:
- Homebrew App store - listing of homebrew apps for Wii U
- haxchi - Nintendo DS emulator entry point by FIX94 and smea
- hb-appstore - source code for app store console application