I wasn't the mod who added the question mark, but I'd guess it's because it makes a grand and speculative claim about the two most hyped topics of recent months. Nothing wrong with that, but in an HN context it generally makes for shallower and hotter discussion, and that is not the best fit for a site like HN which is trying to go deeper. (Note that I said trying, not succeeding.) The question mark is one device we have for dampening that effect sometimes.

One sign of a linkbait title is when it claims a lot more than the article delivers. Usually such articles begin by walking back their own title, since it has served its purpose (getting attention) and is now a handicap (because the text has to deliver on what the title promised). I'm not saying you did this intentionally! But your article follows just this pattern, since by the time you get to the meat of it, the claim has become weaker and in fact has been turned into a question: "what if we thought of Reddit as, functionally, subservient to OpenAI?".

Another thing we often do is replace baity titles with representative language from the article itself. In this case we could do that by using your own question instead of appending a question mark to the original title. That is, we could make the HN title something like "What if we thought of Reddit as subservient to OpenAI?". I think I'll go ahead and do that, since using representative language from the article itself is arguably less of an intervention in this case.

p.s. I just put back your original title. It was a borderline case and not worth arguing over (but maybe the discussion was clarifying or of interest to some readers). Generally when there are strong protests about a title edit we just either roll it back or figure out something that people are happy with.