The average person doesn't use ctrl+f/cmd+f; but still, for the ones who do, having to press it twice breaks muscle memory, it draws attention that something is broken or doesn't work quite like everything else.

the type of people that prefer the generic browser facilities over custom search aren't "average people".

Would these "average people" try to press Ctrl+F? If the argument is that most of the public is ignorant to the client-side jumping/highlighting facilitated by Ctrl+F, what purpose does overriding Ctrl+F serve? It seems that people who press Ctrl+F would have an expectation of what should happen next, and people who want custom search would move their mouse to the search bar and click.

Maybe a better compromise would be including a modal or a hint highlighting the site-wide search box without suppressing the native Ctrl+F functionality.

I feel like I'm a pretty average person.

My instinct when I'm looking for something is to Ctrl+F, not scour the UI for a search bar.

Anecdotally, I know people who are not programmers who are the same way.

if you're here, you're not an average person