Seems to work fine here. For example, if I go into the big crop_rec_4k thread and if I type "download" (a pretty generic word) in the search box on the top right, I only get results from that topic. Or, right here in this topic, if I type "search", I only get 3 results (all from this topic). If I do the same from the main page, I get many pages with results from different topics.
I did enable whole-word matching, as I've searched for LED a few days ago and also got matches for "installed" and many other unrelated words. But that shouldn't interfere with the search-within-topic feature...
The search box is visible in Ilia's screenshot, btw.