Well, the detection is broken for KDE and backwards in the XDG implementation (which is also only used as a fallback when the three DE-specific implementations fail, even though all of them actually support XDG so having separate implementations is pointless).
Also with the way it's implemented, it will have unexpected results for users who have both KDE and Gnome installed (or at least have leftover configuration files) - if you for example used KDE in the past with a theme considered to be "dark" by this and now use Gnome and have it set to light mode, you will get dark mode GZdoom with no obvious reason why.
Oh and the XDG implementation is also very fragile and will not work on everyone's system because it depends on a specific terminal utility being installed. The proper way would be to use a DBus library and get the settings through that.
And when somebody comes to fix it, they will have to figure out a) what's so special about the DE-specific implementations that XDG wasn't enough (they might just assume that XDG isn't supported widely enough), b) learn how to detect dark theme properly on the DE they're fixing, c) rework the code so that there is a difference between "this DE wants light mode" and "couldn't figure out of this DE is in light or dark mode" - both of these are now represented by the "false" return value.
I don't think a well written and functioning code made with AI assistance would get a response this strong, but the problem here is that the code is objectively bad and its (co-)author kept doubling down about something they probably barely even checked.