I had the same question. So I dug into the download page source and found an itms-services:// link pointing to a plist. This type of plist gives info about an app and points to an IPA which contains the app bundle itself. I've worked with enterprise certs before, and this was one of the ways we would initiate the intalls from a webpage.

So my guess is that they are indeed using an enterprise certificate. Though I can't be 100% sure, I can't think of anything else that would be done this way.

I was always under the impression that Apple required companies to have at least 500 employees to qualify for an enterprise cert. But, it looks like they might have lifted that restriction, which is fantastic.

Probably using TestFlightApp: https://testflightapp.com/