This is a bug.  The point is that the code is subverting the protection of its own fcntl locking.  I should have pointed out that Postfix was still using fcntl locking, and that should have been sufficient.  (In fact, it was due to its use of fcntl locking that it chose  precisely the wrong moment to deliver mail.)  Dot-locking does protect against this, but not every program uses it - which is precisely the reason that the code implements fcntl locking in the first place.