To me it sounds like someone who speaks English as a second language, writing well and clearly and in a formal style. It's just unlucky for them that that's the style GPT is so good at too.
I'm a native English speaker, and traditionally when it comes to formal/professional written English (emails etc.) my instincts take me to sounding quite GPTish - luckily I've got a good grasp of the language and have found it fairly easy to alter my formal writing style to be a bit less traditional and a bit less formal too, but if it wasn't my first language and I wasn't a fair bit above average in writing ability even for native speakers, I suspect it wouldn't be nearly as easy to go against how I was taught at school to write in formal situations.
It's really not enough to see that somebody writes roughly in that style to assume they're using LLMs, because the reason LLMs so often sound like that is because they've learned from humans very often sounding like that.
In an example such as this particular case, it maybe set off your LLM suspicions because culturally you wouldn't expect somebody to sound so formal in comments on a site like HN, and choosing the wrong tone of voice for the context is something an LLM is likely to do - but actually, if a) English isn't your first language nor part of your primary culture, and b) you're wanting to make a good impression as the subject of the thread is something you've created and are therefore essentially acting as a spokesperson for in the comments, then all of a sudden writing formally rather than as if writing throwaway forum comments makes sense rather than looking like an indication that AI wrote it.