It seems plausible. Mostly I think people should be explicit and clear about their accusations.
Something that doesn’t make a ton of sense to me in this theory is that, despite it being hard to add a new engine (which is not great), it is easy to switch away from Google on iOS. And the big search engines are in their pre-populated list. So it seems Google and Apple have engaged in a conspiracy to keep people from switching… just to the niche engines? That doesn’t make a ton of sense, right? Google is probably not more scared of Kagi than Bing.
My first guess is that Apple lived through the era of confused non-technical people adding a bunch of scam search engines and didn’t want a repeat of that.