It's a cost-benefit analysis, like everything else. Supporting only github more than qualifies the Pareto need for the feature, as does Discord for realtime communication. None of the alternatives you mentioned to Discord, for example, are likely to already have a client installed for the vast majority of developers; with that as a litmus, the choices are effectively Discord or Slack. When you're doing the hard calculus of how to spend your most precious resource (time) in a FOSS project, you have to weigh the costs and rewards. Only supporting Github likely has no statistically significant difference in likelihood for contributions. Similarly, hosting conversations on an unusual platform most users are not already using increases the friction of their contributions, so you choose the most popular platform.
I'm sure this project's community would welcome a contribution to mirror their git in a read-only state somewhere else, because why wouldn't they? Similarly, I'm sure they'd be fine with collaborating on setting up bidirectional chat bots so you can communicate with them as you want.
But to expect these things from a nascent project seems ridiculous. We're not talking about React or Spring here, we're talking about a brand new project who should be investing as much time as possible making their software work, not catering to every potential communications niche.
If you've decided that contributing to someone else's code on Github violates your sense of ethics or privacy, that's well within your rights and I respect you for it, but you must have enough self-awareness to recognize that that puts you in the far extreme of digital ethicists. And that shouldn't come with an expectation that your ethics have been catered to.