Ah, this worked only for me because from experimenting I still had the line

QLoggingCategory::setFilterRules(QStringLiteral("*.debug=false\ndefault.debug=true\n"));

in KDevelop's app/main.cpp commented out, so it would not overrule the qtlogging.ini settings as written by kdebugsettings.
D5666 will make that line no longer needed, so once that is in and a similar patch for the other kdev* sources, that line could be removed and this patch will make sense and people can start using kdebugsettings with kdevelop :)