Would apply to a services back-end to an application?
As an example, the ubiquitous grocery list: the app is functional stand-alone but has a subscription syncing service that allows for sharing lists and update.
It will probably depend on what the definition of "publisher" so only time will tell. The following seems like it could cover a large swath of things depending on that definition:
"In addition, publishers may no longer provide links in their apps (to a web site, for example) which allow the customer to purchase content or subscriptions outside of the app."
Inside your grocery app is a button that says "Subscribe to sync!" Apple handles the credit card processing and takes 30%.
You go to the website, and there's a button that says, "Subscribe to sync!" You pay for the credit card processing & recurring billing expenses, and 100% of what's left.
Apple is assuming/hoping that the amount of people who buy your service through the app will offset the reduction in revenue.
As an example, the ubiquitous grocery list: the app is functional stand-alone but has a subscription syncing service that allows for sharing lists and update.