I think you're missing a big part of the point of the post: Which is that if you self-hosting, nobody can train models on your data, if you're going to use a cloud service, you should use one where you can move data to self-hosting, and where you can trust the vendor.
We do basically guarantee that you won't have your organization's data included in AI training in Zulip Cloud without consent. But yes, we're not ruling out the possibility of some sort of opt-in feature that might be useful in Zulip Cloud.
I'm humble enough to not pretend I know what will be possible/expected in terms of AI technology in 5-10 years, but one could easily imagine some sort of tool trained on web-public channel data in open Zulip communities being a thing that could make sense if done with appropriate consent. If such a thing were desired, I don't think it most of the concerns related to the slack controversy would apply.