While the extension source is available. The developer deliberately make it non licensed which means it he retains all copyrights and it is not an open source by any useful mean. And he ia quite aggressive about that.
I find it weird that both you and TechnologyClassroom in that GitHub issue thread brought up copyright. Even if the developer had used an open source license, they would still have retained copyright. You can complain about the license just fine, but copyright is not relevant to that discussion.
The developer in the r/apple subreddit said that it is an open source and a couple of comments here made that claim also. So I was just clarifying. It is more about
And to be clear about that. That's from github docs
"You're under no obligation to choose a license. However, without a license, the default copyright laws apply, meaning that you retain all rights to your source code and no one may reproduce, distribute, or create derivative works from your work. If you're creating an open source project, we strongly encourage you to include an open source license" [1]
And in the developer last comment in the issue he closed he quoted that also to explain that it is not allowed to do anything except seeing code for transparency purpose.
https://github.com/christianselig/Amplosion/issues/1