In Stripe's case, the docs are all rendered server side and are viewable without Javascript.
I'm not sure if you can hook into the native CTRL + F search tool and see what a user typed (my gut says no way there's an API for that), so I guess Stripe just wanted to track as much information as possible on what people are searching for, even if it makes the user experience a lot worse.
The docs are indeed viewable without JS[0] (in a limited way) but the default experience relies on JS to render text.
We don't render all content on the page at once for performance reasons, which is (as a sibling speculated) the driving reason for overriding cmd+f/ctrl+f by default.
I hope to write an engineering blog post soon about how we build the Stripe api docs, with some focus on the performance and UX tradeoffs at play here.
