At Youtube or Spotify scale this has nothing to do with engineering hours or project costs. It is for sure a power play message.
I suppose it could be some bizarre internal political vendetta against Apple has led them to be unwilling to invest in improving the iOS app, but that seems like an odd strategy.
Netflix remains the dominant player over Apple TV+, for example.
Chrome remains more dominant than Safari.
Outside of Apples specific ecosystem: Do you know anyone who seriously uses Windows Media Player or built-in Calendar/Mail apps? (Outlook is not the built-in).
If your third party app is better and has better social capital: it can kill the first-party app.
I'm just forgetting which ones completely died because: they are history.
Chrome has 77% market share across desktop operating systems & Safari has 8%: https://kinsta.com/browser-market-share/
OSX has 16% market share which would indicate thar its somewhere ~around 50% of OSX users using Safari: https://gs.statcounter.com/os-market-share/desktop/worldwide
The inference being made here of course that all the remaining people not using Safari are using Chrome which is or course not true; but it shows somewhat the fragility of the incumbent software: as Safari is actually quite good for users, chrome has very few feature advantages compared.
A long time ago, during the Flash time, the recurring theme on the Internet was the low quality of engineering in Adobe products.
There was a popular response to that whining which superficially sounded like a rebuttal: "Hey, be cool and always remember that not all of the bad programmers are employed by Adobe".
I guess there was enough of them to fill all the positions at Spotify.