Nintendo themselves, as well as other places. Here are a few sources regarding Nintendo Consoles and Linux.
Free Open source Software:
https://www.nintendo.co.jp/support/oss/
Switch running BSD:
http://nintendotoday.com/nintendo-switch-linux/
Wii U actually uses MULTI, but the game console OS can run and debug embedded Linux:
https://ghs.com/products/MULTI_IDE.html
NES Classic runs Linux:
https://www.omgubuntu.co.uk/2016/11/nintendo-nes-classic-single-board-pc-linux
While the Wii uses IOS, it is suspected to have a Linux kernel (check the file structure for clues!):
https://www.wiibrew.org/wiki/IOS
While IOS (and IOSU, what the WiiU runs) share a lot of ideas from linux (/dev, ioctls, etc.) it's definitely not Actually Linux. It's a custom kernel - this is pretty easy to see when reverse-engineering it (it's full of things that only make sense on the Wii/WiiU hardware - remember that Linux supports many more things, so you'd expect to find code that could be "swapped out" for other device drivers and the like). Another big tell is that the syscalls are completely different.
I get that's a vague answer that's hard to quantify, so here's something a bit more concrete.
IOS-FS is one of the modules in the IOSU, which handles all the different filesystem types the wiiu supports - fat for the sdcard, isfs for the nand, wfs for USBs, etc. etc.
Question: why does this exist?
Linux has full filesystem support already in the kernel - a proper vfs, permissions, and heaps of filesystem drivers using a known API. If this was Linux, it would have made much more sense for Nintendo to add their custom filesystems to Linux, perhaps as a module so it could be closed-source. Not only does the IOSU kernel have no evidence of this, it ends up having *less* features than even a stripped-down Linux kernel.
To top it off, Linux ships with a high-quality fat32 driver, supporting many partitions and codepages - generally regarded as Quite Good. However, IOS-FS is known to include a third-party, commercial fat32 driver (if someone could find the press release about this that'd be sweet). It kinda. sucks. hard? It's really slow, doesn't have partition support, and freaks out often if things aren't Just So.
Final point: under GPL, Nintendo would have to upload most of the kernel source code to that page you liked. however, the Wii U download in there is mostly WebKit stuff for the Internet Browser, no sign of a Linux kernel.
MULTI is an IDE, an editor to write code in (like Notepad++ or Atom - a better comparison would be Eclipse or NetBeans) and isn't actually an OS in and of itself.
As far as I know, there's a mention of BSD in the Switch's licenses page because it uses BSD sockets - you can watch the switch console security talk (35c3?) where The Big Names mention it's an extension of Horizon, the custom kernel also used on the 3DS.
The NES Classic does run Linux though, at least there's that~