I mean, it doesn't really matter if it's garbage or not; it matters that it's what GNOME app developers are testing their UX against, so if you create a theme that changes the metrics drastically from the ones in Adwaita, your theme is going to break those apps.

You can probably get away with tweaks on color, squashing some elements narrower, whatever, but that's not what the post is really talking about. Things that could be called "variations on Adwaita", or "Adwaita if it had a fancy control panel with metrics sliders and color pickers", aren't gonna break anyone's apps.

They're talking about themes that e.g. change fonts on control labels to ones that don't have the same em-widths or x-heights for some characters, and so things stop lining up flush; or which replacing the icons with an icon set that isn't just "the same icon designs but now in monochrome/superflat/etc." but now has new designs, different to the same degree that the iOS vs. Android emoji themes are different, such that your gun becomes a water gun (or your homedir becomes a hand throwing devil-horns.)