The kickstarter update says the editor is now also available for Mac.
For Mac news, it was the App Store version of the EE that was just released. The editor is only available for Windows.
I noticed the page on kickstarter.com got updated in the mean time, but fortunately I still have the backer mail that was sent out to prove I wasn't just making things up:
We are also happy to announce that the editor we used to make Divinity: Original Sin - Enhanced Edition is now available on PC and Mac.
(well, to the extent that a copy/paste could prove anything

)