We use a generic PrefixIndex on the wiki config index, which results in a list of both the legitimate subpages with per-wiki configuration, but also the language subpages containing a translation of the index help page, for example Extension:FileImporter/Data/fr.
I don't see any way to fix the transcluded list, so we need to move either the configuration root or the help page. Moving the help page is probably easier. I propose having Extension:FileImporter/Data redirect to Extension:FileImporter/List of configured wikis. Move permissions require "translation administrator" on mediawiki.org.