Could the site have continued to grow massively month over month for the next few years? If so, the founder probably would have earned more money independently. The HN audience is large, but it's not infinite, so growth could have stalled at some point. That's risk and reward; who knows? And who knows if maximizing profit was even his sole goal. Perhaps it was exposure, or building a real community to help folks start new businesses. Wherever the site was headed independently, I'm willing to bet Stripe can accelerate the process significantly, and that counts for a lot.

but the founder's alternative was hustling every month to get more advertisers, for probably less money than the Stripe deal.

the best deals are where both sides benefit, this sounds like one

No, he's just implying that it's a smart move for Stripe. He doesn't even know the cost of the transaction.