That's because they own the whole stack, from CPU to GUI and support it as a unit. That's the benefit of having a product where a single owner builds and supports it as a whole.
My impression of Oxide is that that's the level of single source of truth they are bringing to enterprise in-house cloud. So, I strongly doubt the innards would ever become customer-facing (unless the customer specifically wants that, being open source after all).
As for why I think Helios will become customer facing: Oxide is a small startup. They have limited resources. Their computers expensive enough to be very much business critical. You'll get some support by Oxide logging in remotely to customer systems and digging around, but pretty soon the customer will want to do that themselves to monitor/troubleshoot the problems as they happen.
Imagine you're observing a recurring but rare I/O slowdown that seems to trigger under some certain conditions, and tell me a competent sysadmin wouldn't want to log in on all the related boxes (client Helios, >=3 server Helioses for the block store) and look at the logs & stats.
Also, Apple is just the one who survived. Previously I'd have thought of SGI, DEC, Sun, HP, IBM, Dell some of whom survived some not.
Those three consumer products I mentioned each provide a platform for a user and business space to floroush and thrive. I expect a company doing something similar for cloud computing to want the same. But it will require some magick: momentum, money, trust. That kind of stuff, and loads of it. (With some big names behind it and a lot of FOSS they got me excited, but I don't matter.)
Or Linux running underneath all the Java-y Android stuff.
Their choice in foundation OS (for lack of a better term) really should not matter to any customer.
Now imagine a multi-million dollar mission critical pile of computers running on premises, and your sysadmin being able to do so.
Oxide is closer to a rack of Supermicros than AWS.
Solaris is still Solaris, as of the latest release last month. OpenSolaris hasn't been OpenSolaris in a while and is Illumos, yes.