These docs must've been published in a rush. There's even an obvious syntax error as demonstrated by the syntax highlighting on <a href="https://developer.apple.com/reference/applepayjs/applepaysession" rel="nofollow">https://developer.apple.com/reference/applepayjs/applepayses...</a> - missing a 'quote' character in front of 'mastercard'.
It's telling that Service Workers and Web App Manifest remain unimplemented by Apple but they are releasing a (proprietary) payments API, as though it's a bigger need.