There was a CfP process and speakers could pick one or more channels they'd like their content to be with (channels are not just individual Erfas too, many streams are run by many different groups together), and the channels coordinated who picks up what talk. At least the channel where I have insight in the process, the majority of talks is by people who aren't "members". So while there are no "main stages", the other channels are open to all (just as many usually are at congress).
EDIT: CfP announcement for reference https://events.ccc.de/2021/11/10/rc3-2021-meta-cfp/
I have no problem about it being left biased. That's was always like that. What makes me sad is how left became "lifestyle left" and consequently the "lifestyle left" became too present in the CCC congress.
A lot of these talks are held by people that are closely related to the CCC. I expect that once the congress is taking place offline again, there will be a official track with selected talks.
Also please note that the ccc was founded in the TAZ headquarter on the table that previously belonged to the Komune 1. CCC was political from day one. I maybe wasn't so present in the earlier days but as "the internet" got more intertwined with politics the CCC followed.