You are correct. There is currently an open issue[0] requesting support for migrating posts, that was opened in 2019 and unsurprisingly has some comments from today pointing out how useful such a feature would be.
To be clear, when you move accounts, your old server will notify the servers of all the people who follow you such that they will silently be updated to following your new account. This is not something you can do yourself at all; you can contact all of those people and tell them about your new account, but then they'd have to take action to follow it.
E: called them tweets