You should focus on this part:
This flag didn't work on my system(RHEL 6). So I used the flag Qt::X11BypassWindowManagerHint.
Namely why it didn't work on your system. The system doesn't have much to do with this, it is the window manager that either respects this flag or not. Bypassing the WM completely is probably a bad idea since you loose features such as moving the widget around.