404 thumbnail handlers are another complex thing to configure so I had that system also include thumbnail configuration.
However I didn't code it to check if uploads were enabled on the wiki. So it defaults on even when uploads aren't enabled. The $wgEnableUploads in the LocalSettings.php was actually hardcoded in (mistakenly assuming it's already on) along with the comment so that you'd know what part of LocalSettings.php to put the config in. That's something I have to fix later.
If you don't want the 404 thumbnail handler or don't have uploads enabled you can uncheck the "Include 404 thumbnail handler config" checkbox at the top and submit to get config without the thumbnail handler config.