> It’s territorial waters belonging to Iran and Oman.

The trick is that it's still an 'international strait', or a segment of water that forms the only connection between two areas of high seas -- in this case the Persian Gulf and the Gulf of Oman. The principle of freedom of navigation establishes that innocent traffic (civilian traffic, and even warships in peacetime) have a right to use the strait to go from one body of international water to the other.

Iran may claim that it doesn't have to abide by that right, but international law is never self-executing. One question to be resolved by this war is whether Iran will ultimately recognize the right to navigation in any settlement (and then choose to abide by said settlement).

International law isn’t worth the time someone spent to write the words. It means approximately nothing. OPEC is a cartel, for example.