If there was a time when they were needed, it seems that is no longer the case. I removed them from the ebuild and ibus built successfully with USE="emoji kde -gtk" and the emoji picker in kde-plasma/plasma-desktop-5.18.0-r1 works fine with it.
iBus's GUI programs are built with GTK.
I don't see how that's related. I clearly stated in the description that I only need ibus for the emoji picker in Plasma, that works fine without ibus's GUI programs.
OK, I understand the situation. A KDE applet uses an IBus's emoji dictionary for its data.
Unfortunately, it is not the use case expected for this package.
This is ridiculous, even binary distributions package ibus in a way that alows emoji support in plasma without installing ibus GUI programs. For example, if you look at the dependencies of Ubuntu's plasma-desktop you'll see it only depends on libibus and ibus-data, not on the main ibus package https://packages.ubuntu.com/hirsute/plasma-desktop .
If the concern is that users will start complaining that they installed ibus and the GUI programs are not there, that is highly unlikely, since gtk is a flag enabled by default by the ebuild. So users must explicitly disable it to get into this situation.
Also if you consider that having kde use flag enabled might leave the user thinking GUI programs will be installed, then leave it depending on gtk. Remove just the emoji to gtk dependency, that will be enough for plasma.