Quote from QObject::startTimer() :

If interval is equal to std::chrono::duration::zero(), then the timer event occurs once every time there are no more window system events to process. 

I guess this should be referring to the "event loop" which is not necessarily related to windows system events. E.g. a QObject that has a thread affinity to a QThread other than main thread, or even those in non-GUI applications.

Generally, IMO, the documentation of QObject should describe its relation to the event loop when it's describing  event-handling related functions and the behavior of the class in absence of a running event loop, like as described in QObject::deleteLater()