(The best part is the "OS made from scratch". Don't know why, but I can't think it's true)
Probably because they don't show it off at all. I'm sure more than a few of us on here have made hobby OSs and have bored people senseless showing off that we've got VESA text mode supported and a rudimentary CLI with like a whole 5 commands!!!1! Oh and they claim support for Android apps so my gut feeling is they've taken a bare bones AOSP build and written their own GUI at most. Probably just made a custom launcher...
Their choice of hardware makes also makes it pretty certain that even if they did write the rest of the OS themselves they're using the Linux kernel* as I doubt Nvidia would release drivers for a closed source no name OS/kernel. To the best of my knowledge they don't even support BSD kernels for Tegra devices, only Linux and Windows.
*Whether you can claim to have written an OS from scratch when using someone else's kernel is debateable
There is a BIG difference between a hobby OS, something like FreeRTOS (or even less powerfull), and something like what they show that clearly use OpenGL and all of that Jazzy stuffs.
What they are calling "OS" is clearly for me just a Graphical Interface that run on top of something like, maybe Qt? or plain X11.
Anyway, I don't buy their arguments, and starting their video by "We knows that some people will say that what we are doing is just crap" is a bit arrogant.