But they're not blocking oil tankers from other countries, at least not ones that are operating legally. The only tankers that have been seized were flying false flags, which makes them legal to seize irrespective of the embargo.

They have been stationing coast guard ships as interceptors to stop other tankers from reaching Cuba. At least one tanker turned away in the face of the threat from the USCG.

The whole “false flags” argument is also a stretch given that these ships are flying false flags to avoid US sanctions. “We’re not embargoing, we’re just sanctioning” is kind of a nonsense statement when we seize sanctioned ships. The warrant to seize “Skipper” was issued because it was carrying sanctioned oil, not because of the flag it was flying.

This is an embargo, enforced with both economic and military strength. Again, the fact that you want to argue pointless semantics indicates you believe the embargo is not defensible.