We're the product of all the information we consume, but forums like HN aren't custom-tailoring what I'm shown to maximize engagement from me.

Perhaps, but the question was regarding whether this forum brainwashes people.

Do you think there might be some variation in the degree of negative effects a forum like Hacker News has compared to TikTok/YT shorts?

I do, but this is not contrary to my point.

No, because reading is more healthy than mindlessly watching short videos. There is only so much content on the front page, it's not a slot machine you can pull for hours like tiktok.

Give it a few months until somebody rigs an open-source video generation model to generate a "Short" from any frontpage article, and then several "reaction videos" based on the top comments... and maybe a frontend UI that turns the frontpage into an infinite scroll of those Shorts... and bam, HackerReels will be born

Do you truly believe zero brainwashing takes place here?

Out of curiosity, how deeply have you considered the question? And, might that depth not be a function of the norms in the forum?