If we have access to the mental processes inside someone's mind

But we don't.

When judging this story, we do.
We know what was going on in this shipowner's mind, because the story tells us.

I'm not generalizing. I'm making a claim about my judgment of this specific case, based on the facts we're given about it, which include facts about the shipowner's thoughts.

What's wrong with that?

As I said initially... I can see arguing that if we allow ourselves to judge this (fictional) situation based on the facts presented, we might then be tempted to judge other (importantly different) situations as if we knew analogous facts, when we don't. And I agree that doing so would be silly.

But to ignore the data we're given in this case because in a similar real-world situation we wouldn't have that data seems equally silly.