No, big corp with millions of users they cannot moderate defeats the purpose of a fediverse.

First of all, no it doesn't. The fediverse is about servers communicating with each other, that doesn't mean all servers, it just means that multiple servers communicate, and if a server is being problematic it will be blocked or limited, as another example a good amount of servers limit or block Mastodon.social and due to spam issues. Threads in this regard is no different, it just so happens to be a much bigger problem than Mastodon.social hence why there is much stronger efforts to defederate it.

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especially when users can already block content from domains they dislike.

No, you are deliberately or unintentionally misrepresenting how domain blocks and indeed blocking in general works in Lemmy (Domain blocks target communities and do not hide users or their content), and even with Mastodon this doesn't solve the biggest problems with threads, that being the extremely poor moderation and the EEE threat which are server wide problems and not user preference issues.