I already suspected the first comment was by an LLM, but deleted that from my reply as it didn't feel like a productive accusation. However, with "that's a fair point" as an opener, plus the sheer typing speed implied by replies, and the way that individual sentences thread together even as the larger point is incoherent, I'm now confident enough to call it.
I actually use assistive voice transcription as I am unable to type well with a keyboard.
[Edit: update]
I use assistive voice transcription because I'm unable to type well with a keyboard. But I'd point out that "you must be an AI" has become the new way to dismiss an argument without engaging with it. It's the modern equivalent of "you're just copy-pasting talking points", it lets you discard everything someone said without addressing a single word of it.
The fact that my sentences "thread together" is not evidence of anything other than coherent thinking. And speed of response says more about the tools someone uses than whether a human is behind them. Plenty of people use dictation, accessibility tools, or just happen to type fast.
^^^ This took me 30 seconds to speak aloud.