Pretty sure that Matrix is Intel's flagship FakeRAID that is shipped onboard with certain boards. Theoretically, Matrix-RAID is supposed to be supported by dmraid. That hypothetically
makes it possible to dual-boot off of a single fake RAID volume.
This came up earlier. Apparently Fedora's more recent Anaconda won't touch certain types of non-hardware RAID out of fear of FUBARing the data on the array.
...which is supported by the fact that you say:
I went ahead and set up the two native /dev/sda and /dev/sdb as a mirrored RAID device on installation (mdadm under the cover).
The way I read this is that what happened was that the board's RAID implementation was ignored by Anaconda and you ended up with a full software RAID that Anaconda created.
I'd personally vote for a software-only RAID if
you're single booting, unless you can afford Hardware RAID. I don't quite trust any FakeRAID implementation. My main problem with FakeRAID is that if your motherboard croaks, your array is lost... unless you had the forethought to buy a second board from the same production run (i.e. with the same hardware revision of the fakeRAID chip) as a cold standby. With Software RAID, if your motherboard croaks, you can even go so far as take the drives and put them into another working machine, boot up with (e.g.) live media, and recover that way.
Of course, true software RAID entails some (possibly heavy) processor overhead.