By threatening nations who receive the tanker, they are leaving it with two options: take it to Syria or create an ecological disaster by dumping the oil in the ocean.
Taking to Syria - they will not be allowed.
Dumping - they all will go to jail for many many years.
However, there is at least one more option and that one will work - take it back to Iran and let it sit there until Mullahs are gone.
Oh, there is another option - take oil to Haifa and let Israelis have it as partial payment for what Iran owes to Israel since 1979.