TBH I've recently felt like that for ~70% of 'top-level replies' in HN, which has slowly pushed me to other mediums (mastodon and discord).
Could just be that the AI 'boom' brought a less programming-focused crowd into the site and those people lack the vocabulary that is constantly used here, who knows.
I'd go out on a limb and say I think probably LLMs made the general population aware of how the "general voice" feels/looks/reads like.
So rather than a lot of people adopting to write like how a LLM writes, the LLM writes as an average of how people been writing on the internet for a long time. So now when you start to recognize how "LLM prose" reads (which I'd say is "Internet General Prose"), you start to recognize how many people are writing in that style already.
I've been in the internet since the early 2000s, I can assure you it does not write like how 'someone on the internet' would write. And when I say that, I mean that for both sides of the internet: it doesn't sound like how 'old school' internet folks would write, but it also doesn't sound like how teens talk either. Neither of these groups write in 'very plain' English regurgitating useless information.
Recent trends/metas in video formats like tiktok and shorts encourage that kind of 'prose', but I haven't seen it being translated into text format in any platform, unless it's written by LLMs.
My point wasn't that it writes like any specific groups, but a general mix-match made up of everyone voice, but a boring average of it, rather than something specific and/or exciting.
Then of course it depends on what models you're talking about, I haven't tried Grok3 myself (which I think you're talking about, since you say "it"), so I can't say how the text looks/feels like. Some models are more "generic" than others, and have very different default prose-style.
Could just be that the AI 'boom' brought a less programming-focused crowd into the site and those people lack the vocabulary that is constantly used here, who knows.