Netflix offers subscription through the app as well. If you offer those and do not advertise your other channels, you are allowed to publish. The important part here is that you have to offer the same subscriptions at the same price between your web version and your in-app version.

But this would require a significant amount of work on our side to support both web and mobile subscription. We do not expect to get any new business straight from the mobile app, we just want to offer an additional free service to our existing and future customers.

It would take us at least 2 to 3 man-month to rework our billing stack so that Apple has a chance to get 30% on some subscription. Which they won't since we really doubt B2B customers will subscribe to $2k+ yearly contracts using in-apps purchases. So it's really spending all that time/effort so that the Apple reviewer feels OK can safely check the little box on his list next to "in-app policy" :-(