Why .join is still necessary when all other thread have finished before the main thread?

Question

Learning C++ multi-threading.
In my example, thread helper1 and helper2 have finished executing before the main thread finished. Howev

Answer 1

Based on the reference, underlying thread must be joined or detached at the time the destructor is called. The destructor is invoked when main exits, and probably assumes that join or detach has been called.

The code should also not crash, as long as the following two lines are somewhere after helper1 and helper2 are constructed.

helper1.detach()
helper2.detach()