… and what about bot traffic? The current platforms are overrun by them but seemingly only by choice. A truly open platform would be a bot playground.
But it depends, as much of the solutions to our current problems, that things are solved correctly on the root. In this instance, trustworthy government and transparent processes that guarantee zero-knowledge and free, equally, genuine speech and discussions.
You seem to be assuming two things. First, that there is an "easy" solution to this which is available as a result of centralization, and second that there isn't a decentralized one.
The first one is pretty hard for a service which is trying to use a gamable algorithm to show you posts from total randos, regardless of centralization.
As an example of the second one, suppose that your feed shows entirely or heavily disproportionately posts/reposts from people you follow and the people they follow, and you can turn off the second one for specific people. Then if you don't follow bots, you don't see bots. Replies show up if either the reader or the original poster follow the reply poster, and by default for the poster of what it's a reply to or if they choose to make it visible to everyone who follows them.
Now bots can show up to post replies, but only one person will see them and they'll be at the end of the list and marked as someone they have no connection with. On top of that, the user's client can do any kind of algorithmic bot detection they want.
Bot traffic is sometimes detectable only at scale.
They why are the entities with scale seemingly unable to detect it?
Think: corollary to Cunningham's Law.