Can someone explain the FUD around WordPress storing AI API keys to me? How is this different than storing Stripe or other API keys? Genuine question.

Not much difference. Though these keys can use up significant API quota, therefore money. Although, encryption wouldn't make a lot of difference - if the site is compromised, the attacker could have gained access to the encryption key as well. It could only help in the case of a database-dump only attack, if the encryption key was in server env variables etc and the attacker wasn't able to access it. The AI API providers need to introduce spending caps like some cloud providers to address such things.