And by "open source" they mean a proprietary product[0] with "personal" (stripped down and highly restrictive license) and "pro" versions[1], naturally.

The old version (which is on github) is GPLv3 apparently.

[0] http://xenko.com/legal/eula/

[1] https://store-dev.xenko.com/get-xenko

They say that on the front page, but apparently they are not, from the FAQ "Currently the source code of the Game Studio is available only to Xenko Pro Plus and Custom subscribers on a corporate basis. If you would like to subscribe to Xenko Pro Plus or Custom, please contact us." and "Under the terms of the Xenko end user license agreement, you can share modified versions of Xenko in source code or binary form only to users who have the same Xenko license as you or higher. Additionally, you must include the Xenko end user license agreement at the root of the shared files."

No, not actually completely open source. In particular, you don't get editor source unless you license the pro or customer versions.

Unreal may not be open source, but having source access at all license levels continues to be incredibly attractive.