At around 19 minutes Artz tries to explain more on the Atari and sandbox environments. He says the Atari side will be stripped-down to the necessities and compares the sandbox side to running Xbox 360 games on an Xbox One through its emulator. Of course, he's a salesman not a tech guy, so who knows how accurate it is:
I've seen that video, he does talk about keeping the customizable "sandbox" seperate from the main Atari UI (ie. not being able to add your Linux homebrew stuff to the Atari menus).
My guess is that it'll be either some sort of virtualization or a jail container for running the Ubuntu desktop so it doesn't affect the main Linux system that runs the Atari stuff. This is good in a way that you can use sandbox mode to make a Steam Machine, Retro Pie thing, Kodi Media center or even use it as a desktop computer w/o screwing up the Atari VCS itself.
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