The point though is that there used to be a «Linux desktop» you could target before the Wayland transition. Fragmentation of an already small market segment is unfortunate.
Sure. I use KDE myself. I actually like the Gnome default UI/UX more, but I'm really not fond of their "our way or the highway" approach when it comes to customization and interoperability. KDE feels more in line with the true FOSS "spirit", and after a bit of customization, fits my needs better than Gnome.
Huh? Fcitx and IBus both worked on GNOME and KDE as far as I'm aware. Now, Fcitx using QT and IBus using GTK helps them feel more native on KDE and GNOME respectively, but they would both work.
It seems to me that the issues on that page are Wayland-specific; anectotally, on my random X window manager works fine with Fcitx (except for Emacs, but that's probably Emacs' fault, not the IME protocol's).