For me, a "walled garden" is also something controlled and regulated by a party I have no influence over. They can arbitrarily make and change rules and decide

For me, a "walled garden" is also something controlled and regulated by a party I have no influence over. They can arbitrarily make and change rules and decide who to let in or not, which feature to drop, how the UX looks like etc. IRC is an open, community protocol, and as such not a walled garden. Even if I'm not involved, which I could be, I trust the composition of people there much more than any single commercial actor. The power dynamics are fundamentally different. A single instance (an IRC network) may be a "walled garden", in control of the group that runs it. The incentives are different. Also because people can simply migrate to another network given the open protocol (and different, third party clients; the API cannot be shut down like with Twitter/X). Historical example e.g. Freenode ownership change.