I tried to split an artist that had no relationships, but got dealt the following text:
"There are no recordings, release groups, releases or tracks credited to only Sophie Sugar vs Sunlounger. If you are trying to remove Sophie Sugar vs Sunlounger, please remove all existing relationships instead, which will allow ModBot to automatically remove this artist in the upcoming days."

As it turned out the artist had one incorrectly credited track related to it (corrected in http://musicbrainz.org/edit/17677700 ), which made this artist use an alias. But the text above doesn't say anything about aliases blocking split edits. As relationships and aliases are under different tabs, it's not an obvious thing to miss.