This is weird. MediaWiki is requiring interface administrator permissions for editting that page as it thinks it is a sitewide CSS/JS/JSON page. @Nike: Could you please take a look? Thanks.
I've asked around and this is because that message supports Raw HTML and, as such, it's also protected so only interface-administrators can edit it. MediaWiki and Meta-Wiki docs should be updated to reflect that this messages are also covered by the enhanced restrictions, as the MediaWiki messages covering that topic as well IMHO.
Edit: docs have been updated.