Recent posts widget shows the last published(approved) post in any of the topics.

But this post is in an unapproved thread. I think it's right when no posts from an unapproved topic are visible. Don't you think so?

The first post of the topic(content of the topic) is still a post, it's the first post but it is a post. And because of that, if the topic's first post is not approved, but has another post which is approved, the widget would show the approved post.

An approved post would be shown in the widget

But this post is in an unapproved thread. I think it's right when no posts from an unapproved topic are visible. Don't you think so?

PS: then why do you see a 404 error when you click on such a message? If you say that approved should be displayed, then you can navigate through them. Right?

An approved post would be shown in the widget, as the topic is not private, or the post is not unapproved. You can set the topic private, so other users couldn't see the topic if they don't have access to private topics.