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What are they using to determine revenue of the companies?

The source is IndieHackers which is owned by Stripe. I'm assuming the startups must register and opt-in to have their revenue numbers published?

Yeah, you need to connect to your Stripe account when you create your product page on IndieHackers for it to track revenue.

Why would you want IndieHacker to track your revenue? I would thought that Stripe would provide the necessary information already.