> and as long as Ubuntu uses apt and .deb, it's Debian
No, there is more to a distro than them sharing a common package manager/format. The kernel, which is heavily patched by canonical, differs from what Debian ships. Canonical also has a bad habit of carrying a lot of non-upstream patches for other core system components. Ubuntu may have been derived from Debian at one point, but they are fairly different at this point. So much so that you cannot take a system running Debian and 'upgrade' (or downgrade, depending on your point of view) it to one running Ubuntu.
A distro, for the most part is 1) package manager + repos, and 2) system organization (where things get installed, where configs are, how services are managed etc). In that respect, Ubuntu is still Debian - most packages are vanilla Debian packages, and they both use systemd.
Actually, you can smash Debian & ubuntu together, you just get a terrible monster called frankendebian. As a youngster I tried this, it didn't pan out.
> So much so that you cannot take a system running Debian and 'upgrade' (or downgrade, depending on your point of view) it to one running Ubuntu.
That doesn't say much, as sometimes ubuntu and debian upgrades are far from smooth.
As a long Debian & Ubuntu user, I believe you're exagerating the work invested by Ubuntu to custonize Debian. Betond the default desktop environment and attitude regarding proprietary drivers, the work done by Ubuntu is rather negligible.
No, there is more to a distro than them sharing a common package manager/format. The kernel, which is heavily patched by canonical, differs from what Debian ships. Canonical also has a bad habit of carrying a lot of non-upstream patches for other core system components. Ubuntu may have been derived from Debian at one point, but they are fairly different at this point. So much so that you cannot take a system running Debian and 'upgrade' (or downgrade, depending on your point of view) it to one running Ubuntu.