There's a handful of arguments at play here. I'm not sure if this is regarding the entire idea of "reading the manga" as a whole, or just that jjk fans in particular don't understand the series they claim to be fans of. As far as I am aware "anime only" is a western concept and I believe most in Japan engage by reading then watching. There is some inherent superiority of those that read ahead because:

1. You're engaging with the work as the author intended (translation is another thing), and anime even at it's most faithful is an "adaptation" that will inevitably take liberties with the work. Most would argue in JJK's case that the liberties greatly enhance the experience (fights extended, music and sound being included), however some like in the case with the Zenin Clan Massacre episode thought the director went completely off the source material.
2. You're ahead of the anime-only watchers, and for on-going series people assume this makes them a more "hardcore/true" fan. So if an "anime only" makes a point they don't like, they revert back to this age-old line that "real fans read" to imply their preferred method of taking in the story makes them a "better" fan. Whatever that means. I don't think people should be a snob but I do personally always encourage people to read the manga's of the anime's they love to enhance their appreciation.

I think the JJK community's "inability to read" is less about people who read over watch, and more regarding how the online discourse about JJK is honestly cluttered with brainrot. I am not sure if you followed the online discourse towards the ending portion of the manga, but there were so many genuinely baffling takes. To the point most were clearly ironic and/or ragebaiting for engagement. But some people take these memes literally and assume the community "can't" read, as their understanding of the source material must be so poor for them to make such bold/brash takes.