One day they got an email from the CEO of Stripe, asking if they'd want to sell. The request took Courtland and Channing completely by surprise.
Every new business that takes place online and does well is a potential user for Stripe. If Indie Hackers do well at growing the business community, Stripe will do better.
Indie Hacker team gets to use Stripe resources to grow their business, like design, legal and marketing. Otherwise IH is still pretty much independent.