This is a problem that needs to be solved in core, but it is nice to see that there are plugins trying to solve it. Still, searching the DB, which is most likely what the plugin does, is an incomplete solution like your example shows.

What is realy needed is to search usage in the generated HTML, this will help compatibility with more plugins, especially those that store the attachment ID instead of the URL. It still not going to be enough for attachments used on admin side.
Maybe it should use the webserver log as a real life indication for what is used.

But the real question is, what is the use case, and since it is a paid plugin, what is its ROI? storage is cheap and the amount of work that will go into verifying the images most likely will cost more than the extra cost of the storage.
Maybe the "find where an image is used" part is worthy even if can't be 100% complete, the "remove" part probably pointless.