After doing some more research, I think I answered my own question: the overlaid, transparent iframe contains the attacked site, which is usually accessed. For example, it could be a page with a button for deleting photos, emails, etc. The visible page from the intruder’s site is something completely different, perhaps to enter the contest, with a button located exactly where the iframe button is located. So, this does not apply to CSRF, because the submitted form is not fake; it is real in every way and has not been changed.
The solution is to prevent the site from loading into the frame, which is what X-Frame-Options: Deny does.
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