People erroneously conflate modern and flat. Flat is used as a bogeyman.
Plenty of old icons are flat, and modern icons are moving away from flatness. The GNOME-y icon they showcase is clearly 3D, and multiple flat ones they show they state they disapprove of - 3/4 of these are flat and disapproved of.
Saying "ideally, app icons should give the impression that the app is actively maintained, not an abandoned mid-2000s project" doesn't mean "make it flat or else"