Earlier quoted context omitted.
The “don’t theme my app” thing just tells me that GNOME/GTK+/Adwaita aren’t built in a way that gracefully handles theming, or that devs building apps with them are doing things like hard coding colors when they should be using dynamic system colors such as those provided by UIColor[0] in UIKit. [0]: https://developer.apple.com/documentation/uikit/uicolor/ui_e...
Supporting themes imposes an ongoing technical burden across the whole ecosystem. It's not worth it. You don't hear people say "I hate macs because I can't make my title bars purple".
Since customization where pretty standard back then, it feels like product managers pushing their "vision" rather than technical difficulties preventing that nowadays.