I'm not speculating about purchase price because it's more about what was the best alternative for IndieHackers founder but it's doubtful Stripe cares about the site's revenue, today or in the future.
IndieHackers' revenue is ads and the amount they make ($6k / month) is nothing for company like Stripe.
The article says that csallen will no longer spend his time looking for advertisers. That means that either someone at Stripe will take over that role (doubtful) or that they'll remove the ads when the current contracts expire and loose the revenue stream for the site.
My best guess is that Stripe is profitable enough that they can run the site at a loss and they hope for the best case (but hard to quantify) scenario where the site serves as an (subtle) advertisement for Stripe services (and it has to be noted that the site already was a subtle advertisement for Stripe given that many of the interviewed companies already use Stripe, so the site would promote Stripe even if Stripe didn't pay a cent for it).