I'm curious how they did it. Is there a video out there on how that's possible?

You don't need a good battery to swap. The point of the service is to swap yours with poor battery life. They send you a refurb from a previous customer, replace the battery in yours, and then send that to a future customer.

>How? This seems only for people with good batteries that want to upgrade

No? The "Here's How it Works" section specifically implies they're replacing the batteries.

>we'll send you a pair of genuine AirPod buds with restored battery life that have been cleaned and sanitized.

There's no upgrade. You swap your Airpods for another set that has had the battery replaced.