Classic Editor Search Results

I wanted to use this plugin to find all the posts that are on my site and still using the Classic Block. It found hundreds, but not all of them are using the classic block. Some of them have already been converted to Gutenberg blocks, and some of the results were “nav_menu_item”s, which doesn’t seem right, and can’t be edited when you click the “edit” button, and 404 when you click the “preview” button.

What exactly is the plugin searching for when it looks in post content to find the classic block? I see that classic content doesn’t have any <!– type code to search for.

Something doesn’t seem quite right.