I tried to explain that in my own video at https://www.youtube.com/watch?v=8mSyMCJlSwA. This documentary did not really go into that. But in summary they don't have servers to log into where your ID is kept. You own your identity/profile and that stays on the client side. Every post is cryptographically signed so it is your post that no-one can alter. You send posts to a usual default of about 8 relays that pass posts on to who is following you. If one relay blocks you, or disappears, there are hundreds of others. So there is no server to delete your posts. But don't lose your public/.private key as then you've lost your access to your profile. There is no password reset by a server owner. They actually do have a bridge relay with ActivityPub so I was able to follow and reply to some Mastodon users.