He has said after the game. They wanted to get the game launched, early bugfixes, etc, and then follow it up soon with the language/engine, initially closed source, but eventually open source.

Just a guess based on the way he's spoken about it, but I'd expect an open beta in December-February. I'm not expecting that to be open source though (maybe the engine will be but not the language, unclear).

It’ll be a few months further down the line. The developer has said their first obligation is to fix any bugs found by the people who bought the game, so they wanted to leave space for that process.

He said that he foresees the effort to deal with open source community will be huge so I think he’s apprehensive of that effort