IBDs are updated the same way as other distros, it’s just that they’re released as a snapshot.

No. If you take Tumbleweed as example, it also gets released as snapshot. But, IBDs are referencing one image and then copy that, while on traditional package managers

How is the base image updated though? By applying updates to software. There's no magic. You're just doing that on "one" system and cloning it to others. You gain homogeneity downstream, sure. But it's no easier for the upstream developers to find/fix bugs than with any other distro necessarily.

Somebody upstream still needs to be sorting out the bugs and security fixes. The advantage here is that I can run older versions easily because they are complete snapshots. Rollback/rollforward are the major advantages of an IBD to me as it allows you to easily test an upgrade to see if it works and then revert if it doesn't. You can get similar functionality with fancy btrfs/lvm/zfs snapshotting as well, but it's different and may require some effort to restore.

every [non-IBD] install is individual and drifts over time.

This is the part I take issue with most. If, and I realize this is a significant "if"[1], somebody is using the standard repos and installing packages using the package manager and flatpak/snap/etc. then there will be very little "drift" between libraries in the upstream and on the local system. apt/dnf/yum are quite good at what they do. After doing an "apt update" I'm up-to-date with the current Debian for things I have installed from there. There's nothing magical about snapshotting that and delivering it instead.

[1] You could make a case that maybe I've added a bunch of other upstream repos and that can lead to differences. But you can also make the case that people using IBDs can slipstream in package changes as well. If you want stability, don't do that.

I’ve seen many posts and Git-issues where the devs or other users said “closed, can’t replicate”.

Sure? But you seem to be ignoring hardware differences. Distro maintainers have a small slice of available hardware on which to test. I can't test my code on every CPU, video card, motherboard, etc. There could easily be a driver bug that affects some card I have that none of the distro maintainers has access to and so it's not tested. This you see quite a bit actually where sometimes even different revisions of a laptop in the same model/family will break Linux compatibility due to a different wifi chip being used.

There’s a reason why immutable distros were developed a few years ago. Even back than they were very promising, and there weren’t even remotely as many Flatpaks available and technologies like Distrobox and much more weren’t there too. From now on, they will only become better and better!

Personal opinion - I think IBDs will go away and flatpak/etc. will remain. If applications are containerized then they are mostly isolated from the host anyway which is the real benefit. May be wrong though.