If it's harder than that: I expect them to devote engineers and brainpower to the problem until it's not. They should solve it - that's their business.
I tried just now searching for 7 line checkout code, finding the code for it. (It's fine if it's 12, that's not the sticking point - but I can't find 12 either). It's not there.
The simplest result is this one.
https://stripe.com/docs/payments/checkout/accept-a-payment
Note the "client and server flowchart" diagram, with its 6 boxes.
https://github.com/stripe-samples/checkout-one-time-payments...
I think the closest to 'simple install' I can get is this code, which was not easily discoverable, in my opinion.
https://github.com/stripe-samples/checkout-one-time-payments...
It stand by what I said: it's a hell of a lot harder than 7 lines of code. I find the claim irritating because the reality is it's blatantly untrue.