We always want the source that contains the greatest amount of information about the topic. As I wrote in the other reply in this subthread, the heuristic is whether a source contains "significant new information" vs an alternative.
That means, as explained in that reply, an article about the findings of an academic study is better than the academic paper, if it contains significant new information that isn't easily found from the paper itself (particularly if the article contains quotes from interviews with the researchers). A project creator’s blog post about a new project or release is better than a link to the project's GitHub page.
We generally prefer not to link to a third-party's social media post about a project, on the basis that it's light on significant new information and takes traffic/attention away from the primary source or another in-depth article about it. (It's different if it's a 3rd-party's detailed blog post about a project, which includes their own experiences using the project and comparing it with other projects in the same category. But then it's more of a review, than a report about the project itself.) Another problem with submitting a 3rd-party post about a project is that it then becomes a topic of debate in the comments, why one source was chosen over another, which happened here.
In a case like this, the information that was in that social media post could easily have been quoted in a comment in the thread, that we could have pinned.
Given that the author of the project posted an announcement in a discussion forum, there could be a case for making that the HN source, given that it contains the other relevant links and some additional commentary, though in this case it's a bit light on detail. But it makes all the difference that the source we link to is by the author of the project.
In the case of this submission, the story has been on the front page for 12 hours already, including some time at #1, and is still going strong, so I don't think anything has been lost.
You're always welcome to make a case for why a particular source is the one that contains the most "significant new information" and is thus the one that should be the HN source.