Crucial detail:
The backup archives are “stored without a direct link to a specific backup payment or Signal user account,” O’Leary says. You’ll use a recovery key to unlock your backups, but if you lose that key, the company “cannot help you recover it.”
And this reader comment was clarifying IMO:
This solves a user issue of changing a phone and their Signal message history is just gone, which to normal users, is not acceptable.
Keeping them definitely seems to increase possible risks, but for most people this is a requirement, so good move on their part (guessing it's optional) - when it's rolled out.