It has been Slackware based since inseption I think just because that's what fit when it was created.Fedora particually Silverblue and in turn some of the Ublue projects have shown you can create a super stable environment for container based workloads.

Updates are seemless and targetable a fedora backend gives them an easier switch to podman should docker not be viable in the future.

From the article it looks like they're focusing on Faster security updates Better hardware support through the Linux kernel There's a clear seperation of the base os and the apps,plugins etc that Silverblue/Ublue can offer that Debian couldn't do as easily and they'd be better off just sticking on Slackware.

It's in the post. They wanted immutability:

"Unraid has always been a so-called immutable OS— however you modify your server, if you go in there, you type the wrong command, you delete all the files from your root, you still could just reboot and you're back to the virgin release… and that's a big reason why we've chosen Fedora [uCore]."

— Tom Mortensen, creator of Unraid

Because they decided to go with Fedora.

Why yes debian?

When the enterprise OS company I worked for in 2k1 was considering forking a Linux distro to offer as a freemium product, they evaluated enterprise Linux and Debian.

Debian lost then as its package format had weaker validation and consistency guarantees. It'd've been harder to support.

The packaging in both hasn't changed much, except RH forgot how to use /etc/alternatives properly and now has some ungodly rinkydink thing supplementing it like a colo bag on the side, but the validation difference is still there.

Maybe that was why. Both are riddled with fridge art - oh hi Systemd, network manager, et al - but both are generally similar other than packaging and pipeline.