I've witnessed bots here on accounts that are years old with no history that start posting multiple times in a short timeframe suddenly after being dormant forever. Makes you wonder how they're getting these old accounts. It's not just new ones.

Black market accounts. Some human made them years ago for a price, they are sat on by some black market / grey market guy and now he's selling the accounts for a profit.

Old accounts from multiple social media platforms has a $$$$$ value.

Account takeover is a thing that happens. I doubt HN has the best measures against credential stuffing.