A whole this voting / flagging system here is basically lottery.

I can't write more than 5 posts per day and I can have negative karma because people doing more downvoting than upvoting.

So there is no point to wonder, if it's coordinated trolling or not.

I didn't downvote you, not sure what you are talking about.

Is this AI written?

I didn’t accuse you of downvoting. It’s literally not possible for HN users to downvote the comment they reply to.

Anyone with seemingly over 10k karma would know that.

I genuinely didn't know :-). I don't generally downvote comments I reply to. I don't downvote when I disagree, I downvote when I believe the comment is very low quality, in which case I don't engage at all.