Their method seems really efficient to me. It infinitely scrolls, feels like it's actually native, and hijacks the shortcuts to make them work as if it was native.
I really like their timeline scrollbar as well which lets you easily move through hundreds of posts very quickly and has become a pattern in many apps like Google Photos and Telegram.
It does not hijack the shortcuts to "work as if it was native", because there's no way to know how the native function works in every browser, in most cases.
Let's look at Ctrl+F again. The standard Chrome search toolbar is search-as-you-type, highlighting occurrences immediately. It also shows the total number of occurences found on the page, and has arrows to navigate to next/previous. It also doesn't auto-hide (and thus lose focus) if you scroll the page.
What does Discourse replace it with? A search toolbar that requires an explicit submission to even start searching - and then, instead of actually scrolling to the occurrence of the search term, it shows a dropdown with snippets of posts in current scope that matched the term, highlighting it much like a search engine would.
So it basically has nothing to do with the native function that it hijacks, other than the broad concept of textual search.
