The event was prerecorded. It doesn’t seem farfetched¹ to think they already had the approval but legally couldn’t say it without the FDA making it officially public. But if the FDA had announced it before the event, it would’ve stolen the surprise. Not only of the feature, but that new AirPods would be announced. Apple would’ve hated that so they may have asked for the announcement to be delayed.

¹ Maybe it is. I’m not an expert on the USA’s health and government policies.