How? I don't even think decentralized is the appropriate term. They're distributed entertainment platforms in that they're protocol based, but regarding the distribution of content there's nothing in it that decentralizes reach. The social graph of Twitter and Mastodon could in principle be identical.
Malicious actors don't need to control algorithms. States running influence campaigns on say, Youtube or Facebook don't actually control any algorithm, they adapt their content to what does well on the platform. And they could equally do this, one could argue even more effectively, on the fediverse.
Saying the Fediverse solves top down influence is like thinking that Bitcoin solves wealth inequality. The distribution of the network is completely agnostic to the centralization of the content.