But those take a ton of time to create, and a ton of skill to write well, and I feel like up and coming niche projects that need to build a community naturally would emphasize Discord over docs.
One way to think about documentation is that it is useful to help scale up support. But if there is no scale, it is less important.
"Design and build software, even operating systems, to be tried early, ideally within weeks. Don't hesitate to throw away the clumsy parts and rebuild them."
If anything, it is a lot closer to move fast and break things.
The group members have the freedom to use any free and open source client or one of the popular web interfaces to join the channel and talk to each other. Should something go wrong with the Matrix or IRC network (say, the network becomes user-hostile for some reason), it is a simple matter of moving the channel to another benign network. The group members only need to point their clients to the new address.
In fact, such a thing did happen once in 2021 when Freenode went through major staff changes due to disagreements with how the network was being run. All channel operators (myself included) moved their channels to Libera in a matter of days. It took me only about 5 minutes to complete the migration and hand out the new channel address to the channel members.