I don't think Apple could get a percentage of the revenue which would not go through their in-house payment system.Yes, but it says licensing fee or comission. And a percentage of revenue is basically a type of commission, right?
I mean, before one can earn a cut as a percentage of the revenue, they have to know the total revenue. Since this revenue does not go through their payment system, they would have to ask the devs to fill in "tax forms", and there is no way Apple could ensure that devs tell the truth. They don't have the power of government to fight tax evasion. Even if they did, keep in mind governments struggle with tax evasion.