I wouldn’t update to 13.1 beta just in case it isn’t actually gonna be released with the new phones. Your iCloud backups cannot be restored to the new phones when you get them on launch day if they only come with 13.0 and if 13.1 is not released at at that time.
What I read from article is that even if you are on 13.1 dev beta, can still receive iOS 13GM an update to it, then continue to get iOS 13.1 dev beta 2, 3 and so on.
