PSSM11 — Checking the completion of doActivity

Currently the specification says: "Completion occurs when, after a run-to-completion step, there are no more event accepter registered for the doActivity with its DoActivityContextObjectActivation".

However, cannot a long-running action still be executing even when there is no accepter registered?

For example:

init -> fork -> accept -> join -> final
-> action with computation

The accept can accept an event and move to the join (run-to-completion step performed for the doActivity), but the action with computation can still be running on its own thread (according to the Threading Model section in the fUML specification). What guarantees that there are no more concurrent parts still executing when there is no accepter registered (there is no "default" accepter for a doActivity as far as I understand).