If they make modifications to GPL programs, they must publish those modifications. But if they don't modify the actual programs, there is nothing they have to publish, they can just point to the official source code and say "that's the code we're using", it's published. (* Disclaimer: I think there are actually some legal questions around what exactly constitutes the requirement to publish the source code that have never really been and probably never will be actually argued in court because they're effectively unrealistic and this is the practical reality, but IANAL and I am not modifying any GPL code without actually publishing it so I don't really care)

The key thing to understand here is that an OS is not a modification to a GPL program, it is simply a collection of them. While there are enough of them that you can create an entire OS out of entirely GPL components... importantly, you don't HAVE to, nothing in the GPL says they are only allowed to exist alongside other GPL software and components, otherwise you couldn't use them on Windows for example. When you're making an OS, any component you don't want to use GPL code for, you just don't use a GPL component for it, you write your own that does the same thing and fills the same role for your OS. As long as they are doing that from scratch, not by modifying the GPL original, they can have as many proprietary programs in their OS too if they want, even required parts.

The proprietary components might be required for their OS to function, thus making the whole OS proprietary, but it can still be mostly GPL code, as long as they haven't modified the GPL parts and have written the proprietary parts from scratch or with appropriately licensed libraries.