One answer to that is that the sending server should send the activity to every actor on the ActivityPub network. That's clearly too much -- literally millions of actors. There's also not a big list you can look at that shows every actor on the network. It'd also be annoying to receive every single public post in your home timeline.
Another option is that the sending server should send the public activity to every *server* on the network, instead. So, if there are on average N actors per server on the network, it's (all users)/N, which might be tractable, although it's still pretty big on the modern Fediverse (~30,000 servers, according to fedidb.org). But, again, there's not a big list of all the available servers that the sending server can use for publishing to.
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