Another open source option is Synology's Disk Station Manager which includes a bunch of open source software they've skinned and created mobile apps for so you can localize stuff like music streaming, dropbox, notes etc - https://www.synology.com/en-global/dsm/live_demo
I cannot find the source code of DSM. Are you sure it is Open Source?
If it isn't Open Source this is off-topic since there is a large number of other commercial NAS OS e.g. Thecus, SoftNAS, QNAP, Napp-It... Most of them tend to have an "app store" and some people building more or less working apps out of Open Source and free projects.
Your link is people complaining that the source isn't being released fast enough... and Synology has told them it will be put on SourceForge when they're ready. There seems to be a lot of precedent for that to go how they said it will.
There's also an independent community that facilitate installing and using it on your own hardware including the latest version that's still waiting on the source dump.
Sure, no one needs that source code anyway because they are not adding anything of value. But if they are not complying the GPL license they are legally not allowed to use the GPL projects.
Yes, on x86 systems it mimics a DS3615. I've seen it installed on cheap dell desktops, like an optiplex 755.
Works well, with some limitations. I dont think their automatic port forwarding service (quickconnect) works well - since that service checks for a valid Synology MAC address.
Additionally - you can't do the automatic system updates as it might break certain portions that XPEnology overrides.
The code base isn't open source. They modify the linux kernel so that is available (or will be soon). But Xpenology simply only allows you to run their closed source code on any computer.
