Isn't synology running it hybridized with their own proprietary storage layer?

It's not proprietary, you can hook up the drives to a linux box and pull files off them just fine if your box dies. I've never had to do this, but I've seen others say they've done so successfully.

I believe it's a layer of mdadm and lvm rather than anything proprietary.

I could be wrong, but i think i remember that they have made some "magic" inside mdadm or lvm to allow btrfs self healing to work, and bypass the normal lvm self healing.

They tooted something around the release of DSM 7 that Btrfs now used native self healing instead of relying on lvm, but of course i can't find any documentation for it.