Here's what I think happened:

The ship was hit by a missile-type object designed to do minimal damage. The point was to get the crew spooked so they would get off. The mine was placed there to be detonated after everyone had evacuated. The explosion would have made fantastic visuals for the news, and gone a long way to tilt world opinion against Iran.

Except that, as reported, the evacuating crew told Iranian navy patrol (not IRGC, but same type of boat) about the undetonated limpet mine sitting there, and the Iranians removed it.

The ship was being filmed the whole time, possibly to provide footage to the news after the explosion. The mine was placed above the water line so it can be detonated from afar with a projectile. After the Iranian navy disrupted the operation, the footage of what was supposed to be the explosion was provided instead as proof of Iran's role.

They were banking on cinematic visuals to manipulate the media, and it didn't quite go as planned, so they're trying to milk it anyway, and badly failing.