See: why authors wait days, weeks, or even months before editing what they've written (or, if you're more interested: cognitive regression, inattentional blindness, and the effects of misdirected saccades).
I read this to mean he wrote the comment, then asked Claude to fix the grammar (as many ESL speakers do). Sounds to me like he did write it.
2. I'd wager the majority of HN commenters don't read their own comment before posting (pre-LLM days).