QUESTION: clockwork will skip execution of the task with the :at option

[pdkproitf]

I read through the docs, it seems conflict between at: and thread:

• at:
  If another task is already running at the specified time, clockwork will skip execution of the task with the :at option. If this is a problem, please use the :thread option to prevent the long running task from blocking clockwork's scheduler.

• thread:
  By default, clockwork runs in a single-process and single-thread. If an event handler takes a long time, the main routine of clockwork is blocked until it ends. Clockwork does not misbehave, but the next event is blocked, and runs when the process is returned to the clockwork routine.

I did some tests and I found the task still running after the process is returned to the clockwork routine. So Am I misunderstand some points or the documentation isn't right?

Is it should be If another task is already running at the specified time, clockwork will block execution of the task with the :at option