Really? Even the macOS client is Qt? I was fully convinced that it was an AppKit app, it looks so good and is so responsive. That's extremely well done, IMHO.
For mac they have two desktop apps, one native and one in Qt.
Do you have an idea why they are doing that?
Why would you maintain 2 targets doing the same thing for an OS?
Maybe because the Qt one doesn't need any special Mac specific code. It's just another build target of the windows/Linux app
They've maintained Telegram and Telegram X for Android for a couple years now too, and for almost a year on iOS until X replaced it. This is far from a new thing for them: https://telegram.org/blog/telegram-x