That's true from a technical perspective. But in reality devs (especially ones who aren't making "Linux apps" but are doing things like porting a previously Windows-only game to Linux) will occasionally ship a broken Wayland client. The compositor could then still give that a basic titlebar with window buttons like KDE and Cosmic do, or alternatively it can refuse to do it and make the novice user annoyed at the system as a whole.

I'm not really convinced that requiring all Wayland clients to draw their own decorations was the correct decision in the first place, but even if we accept it, I think there's still a convincing case for fallback SSDs.

so that’s just a broken app

technically, yes. but at the end of the day, in the real world, that app would work fine on ~all but ~1 desktop compositor

wayland doesn't require SSDs, because it's not just meant for desktop computing. in the context of desktop computing, it's been a standard for decades now to have a title bar on a window, so it does make sense that apps would assume they can get one without having their own drawing code or relying on a third-party library!