CTRL+F hijacking is necessary in some cases when apps are not displaying the full text that the user would expect to search. E.g. when there's a 10K-line code file and the UI is not loading the whole thing into DOM, but the user would expect a "find" to search that whole code file.
Browsers can deal with very long documents. Ctrl+F works like a breeze on HTML that's 100K lines long.
Browsers only struggle to run heavy JS frameworks that wrap every line in a dozens of spans with dozens of handlers and mutate it all on every line scrolled.
This misses the point. Websites are allowed to replace default keyboard shortcuts for a reason. There are only a few exceptions to this, like Ctrl+W. In other words, you can design your website however you want, except to make it more difficult to leave. This is an implementation of the same philosophy.