I see it as more of an extension than overriding the function. Ctrl+F means “I am looking for a specific string on this page”. But if the text is in the topic but hasn’t been loaded in, you won’t find it, but the search will.
I think it’s unreasonable to expect the user to understand the pagination implementation of the endless scroll. So looking for a string that is 100% known to be in the topic but not finding it is highly undesirable result. I prefer they are nudged in the correct direction.
The 2x Ctrl+F is also good because many times I do just want to do a native text search