That’s only mostly true and more importantly not what this is about. Yes Gnome and Mutter don’t support server side decorations. But Electron on Linux uses GTK to construct the application window. And GTK offers client side system styled window decorations. Meaning that electron applications aleady supported decorations that look and feel like server side decorations even if they are not.

Electron already had some support for client-side decorations, provided by a class called ClientFrameViewLinux which uses GTK to paint convincing native window frames. These look very similar to the ones GNOME used to supply on X11, but they are produced entirely in-framework.

No, the problem is with custom styled window decorations. Developers who wanted to do CSDs couldn’t without major downsides. And that was also true on KDE Plasma, as evidenced by this screenshot from the article you evidently didn’t read

See how the window for VS Code doesn’t throw a shadow compared to Dolphin? That’s because electron didn’t support CSDs properly. And now that it does the window looks like this:

That’s what we are talking about.