In terms of packages, Pop!_OS has the flatpak over Snap, which I believe is not installed by default. Their distro is also known for a brand new tiling window manager on top of GNOME [0]. They have also recently made a notable contribution to improving the "Linux on the desktop" experience (e.g. [1]).
I would not oversimplify it as just "repo added on top". For many years we could say the same about Ubuntu and Debian, but I don't agree with this generalisation.
It may walk like a duck, but if it doesn't quack like a duck, it might not be a duck.
What does that even mean? Is Ubuntu just Debian with another repo on top? Either way, it doesn't change the fact that Pop ships with different functionality and is controlled by a different company that has made a lot fewer user-hostile choices than Canonical in recent years.
Ubuntu tends to fork Debian's unstable branch then freeze it, patch it, rebuild it and call it their own. Pop_OS! is simply adding some software on top of Ubuntu.
If Debian makes a change, Ubuntu is unaffected until they resync. They then have time to patch it and test. If Ubuntu changes a package, that change is immediate in Pop_OS! unless they ship their own version of the package.
I agree that Pop_OS! is doing a great job with their DE modifications and other defaults, but I don't view that as them being in control. Unless a lot has changed since I used it last, Ubuntu and their infrastructure is still building most of the distribution.
A lot of this is really where you want to draw lines in the sand. What makes a distro a distro and not just another derivative?
Unlike Ubuntu, Pop doesn't really have full control over their users' experience, because their users are configured to consume directly from Ubuntu's repositories.
They are clearly trying to build a walled garden. The Pro stuff, making security package updates subscriber-first, clearly makes everyone else a target.
I can see why they are doing it, commercially, but it seems misguided. There are so many other ways they could bring value for corporate customers without trying to copy Apple/Microsoft/Red Hat.
I would not oversimplify it as just "repo added on top". For many years we could say the same about Ubuntu and Debian, but I don't agree with this generalisation.
It may walk like a duck, but if it doesn't quack like a duck, it might not be a duck.
[0]: https://pop.system76.com/
[1]: https://blog.system76.com/post/more-on-cosmic-de-to-kick-off...