It would surprise me if a lot of Twitter support people had access to tools that allowed them to post tweets as another user. That's not functionality that should be available to a Twitter support person.
People's accounts get hacked all the time. To help them recover is often a manual process, because the true owner of the account can become unclear. To be able to do that a support worker must be able to change the email address on an account, undo 2FA settings and make other changes because hackers will typically change the email address and add 2FA of their own phone as the first step in an account takeover.