When the Linux engine was finished it was quite out of date, with the delays due to middleware, etc. There was the release branch of the game, which was more up to date, though of course didn't include any of the things still being worked on (harder difficulty mode, new skills and abilities, etc). That work in progress branch of code is what will become the Enhanced Edition.
The Linux version is skipping the D:OS release, and being updated to the most current version for the Enhanced Edition.