Why? For the same reason any story floats to the top. We are clicking through to read the linked article, up voting, and (like you) participating in the discussion. Put all this stuff together, and the story will trend highly for a while.
HN doesn't have the kind of purity filter you are looking for.
You can have whatever dislike of Apple products and their walled garden that you like (I certainly don’t own an iPad myself), but at the end of the day Procreate is one of the most popular digital art apps and many artists perceive generative AI as stealing their art, so the anti-AI stance Procreate is taking is definitely discussion-worthy on HN.
https://news.ycombinator.com/newsguidelines.html