ApiSetItem needs to verify that the target site is in the correct group, that is (for now), that the site is a wikipedia. The same goes for ApiSetSiteLink.
This was previously done by only listing pages in that group as possible values for the respective parameters, but due to the changes to the Sites interface, we are now again accepting all target sites, including wikibooks, etc.
Version: unspecified
Severity: normal
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=38263