After adding a 2FA authenticator, the second-factor page shows correctly the added authenticator.
The Two-Factor Backup Codes sections only says:
You must enable a primary two-factor method before generating backup codes.
It’s necessary to leave the profile page and enter it again (or reload the page) to see the “Add backup codes” button.
Unfortunately, I can’t upload a screenshot here as the software always says “Sorry, but your upload failed. Please try again.” (the POST request to https://meta.discourse.org/uploads/complete-multipart.json
returns a 422 with also just this text as JSON’ed body). But the content of the screenshot isn’t very enlightening anyway.