Earlier quoted context omitted.
In practice, this is rarely an issue due to the nature of human attention. Beyond a couple dozen speakers in a conversation, it's noise. At least to my observation; I haven't pulled apart the protocol to know why: if you're in a conversation in Mastodon it's real good about keeping you in it. The threading of posts seems to route them properly to the host servers the conversing accounts live on.
And yet I could have a realtime public threaded conversation on Twitter, and am having one on Bluesky (regardless of which PDSs or AppViews other people are using), but cannot in principle have on Mastodon (unless everyone I talk to shares my instance). Does this say anything about relative ability of ATProto vs ActivityPub to meaningfully compete with centralized services? I hear your point that slower conversation…
Auto-at-tagging doesn't scale to dozens and dozens of actively-engaged speakers, but neither does human attention, so that's not a problem that needs to be solved.