They kind of intentionally danced around that question with 'are APIs copyrightable'.
Because here it's allowable for a product that isn't competing, and is being leveraged for interoperability/ease of users to adopt (rather than for how clean/clever it is).
What if someone had copied Stripe early on, though? Stripe's big claim at launch was "payment processing in 7 lines of Javascript" or similar. Obviously there were other barriers to entry, but what if someone else entered the market at that time with a similar (or even identical) interface? That question was left unanswered.