Simple. First, Apple is not a monopoly. Spotify has plenty of other options to sell its product and by all accounts is selling a lot more through other channels. Spotify is just trying to bolster its case by charging more for an in app purchase, IMHO. Given the small number of paid subscribers they could just pull the app if they felt the terms were onerous.Spotify doesn't run a app store, so how does this new info from apple invalidate the monopoly and anti competitive complaints? Apple still gets to use iOS notifications to show apple music ads.
What Apple is doing is not different than a B&M store making a profit on the sale of 3rd party goods.