a skeuomorph (from greek, "tool/container-shape") is something that retains the characteristics of another thing that it is based on, even though those characteristics are no longer useful. think lamps shaped like candles, or the floppy disk save icon, or media player programs with volume knobs.
skeuomorphic UX is a good way to get users comfortable with a system by using designs they are already familiar with, and the original iphone used this to great effect.
This is a good example of skeuomorphic UI:
all to say, I'm not entirely sure these icons are skeuomorphs. they're just glossy.
it's weird that. it's obviously possible to have a flat-shaded skeuomorph, just look at basically all of windows 95, but for some reason we connect them to this particular graphical style. files and folders are both part of the old classic "desktop metaphor", so they basically have to be skeuomorphs. but like, the application icons are basically just mosaic tiles of the normal icons.
a proper skeuomorph would indicate what the program is for. krita and whatever map software that is are both good, if a little flat. but the libreoffice suite just being squares with a letter on them? have them be like, a spreadsheet for calc, a stack of cards for impress, and a printed page for write.
remember all the icons for windows 95 network utilities that have people in them? those are also (attempts at) skeumorphs because they're trying to communicate what the program does.