In the article, it mentions several times that Apple has rejected apps for linking to the web page to upgrade, or for even mentioning it is possible to upgrade (when IAP is not enabled).
The article says so but I just tried to watch a video on iOS amazon video and it tells me to “Buy through the Amazon website or on your TV”. So I’m sure Spotify could say the same.
I believe it's about the type of product you're paying for -- the Spotify site specifically says Apple doesn't allow _content subscription_ services to be advertised through apps. In addition, I bet Prime gets away with it because it's not _purely_ a content subscription service.
Can you even subscribe through the app? Spotify could simply yell you so go to your web browser to subscribe. That’s what Amazon does for its video etc and it doesn’t seem to cause them any problems.
I subscribed to Spotify via their web site and it works fine on my phone. I’m sure they pay nothing to Apple in order for me to listen. That makes me feel that this is a publicity play by Spotify no matter what the more general (non-Spotify) merits of the issue might be.