I can't find any difference between KAlarm's behaviour before and after applying the patch. The --help-all option isn't recognised in either case. (I'm building with Qt 5.12: perhaps --help-all is now recognised by a later version of Qt, like it was in Qt4 days?) Other KDE applications on my system also don't recognise --help-all.

That's possible. This appears to be added (back?) with https://code.qt.io/cgit/qt/qtbase.git/commit/?id=341c8b9cd08 last May which is newer than the 5.12 branch point it seems. My kalarm --help (as well as most other KDE/Qt programs) shows (a variant of),

Usage: kalarm [options] [message]
KAlarm

Options:
  -h, --help                         Displays help on commandline options.
  --help-all                         Displays help including Qt specific
                                     options.
  -v, --version                      Displays version information.

which is why I was surprised that kalarm --help-all throws no error and just starts normally as if I've passed in nothing. I assume your --help does not include a --help-all option. Searching the doc this appears to be added in 5.14. 5.13 doc doesn't have it. (Qt doesn't seem to have a stable link for the latest specific version so https://doc.qt.io/qt-5/qcommandlineparser.html#addHelpOption should show the 5.14 doc before 5.15 is released)

Thinking about it, I think this is an argument to use process instead of handling it manually. Qt clearly make changes to what builtin options there are....