I wouldn’t expect the bookmark to run afoul of this clause, since the bookmark isn’t the content. Now it’d be a curious case if the bookmark contained a base64-encoded pornographic image.

You seem to be assuming competence on the part of the author. But, as is common with documents that lawyers generate, they probably don't care if it's reasonable or if practically every one of their users violates it. Like when you get an employee contract that claims your new company owns every idea you ever had. Some people will claim it's just "lawyer stuff" and is somehow okay. It's really not okay.

And synced history.

If you're syncing a bookmark that is somehow illegal content, it would come to rest on their servers and they'd potentially be liable for it. (IIRC they encrypt everything at rest, so this is a speculative risk)

Depictions of sexuality or violence are legal in most places, even if said depictions are graphic

Porn is not illegal, either are the Rambo movies.