"Do they let certain companies use their own back end payment systems?
Do they let spotify/netflix etc. do the same?
The answer to the last question is: Absolutely yes.
Spotify can and does sell through their own website, and with their own payment processing.
Spotify could also sell CDs, merchandise, show tickets, through their app, and they not only COULD but they would HAVE TO do this without using in-app purchases, using their own payment system, taking 100% of what the user pays.
The exception, same for everyone in the world, is purchases for things that stay on the iPhone. Spotify can just do exactly the same as Netflix, which allows subscriptions through their website exclusively, with not a penny going to Apple.
There's probably a million iOS developers who read and understood Apple's T's and C's and who _know_ that it is Spotify who wants to be treated differently from everyone else.