I am not going to open source the project at the moment, I have to clean things up a bit before that. I'm probably gonna do that as soon as it reaches a good first RC
I would imagine you're going to get a fraction of the users testing it that you're looking for then. I know I'm not going to put my username and password into a closed source app from a random stranger on the internet.
No, GitHub never required your code to be open-source. In the past, free accounts couldn’t create private repositories. Nowadays, they can, but those private repositories are limited.
A public repository does not need to contain open-source code, or even any code at all. I imagine the repo just contained a README, and that’s completely fine GitHub-wise — why would GitHub mandate people to have code in repos?