The way that this is currently implemented is very confusing to the user. if I search this site for the example above, “plantofjazz”, I get this topic at the top (good) and then some other ones below, such as this. If I search one of those others for “plantofjazz” using “in this topic” I get “no results found”, because those other search results were false. That behaviour is at best inconsistent.
I’d suggest that a site-specific search such as the one within Discourse has a fundamentally different job to do than general web search. The job of a general web search is actually to put as many advertisements in my face as I can tolerate, and then then show sites that might somehow relate to my search (typing errors and all). Discourse’s search is different - it’s restricted to one topic, and logged in users are surely much more likely to know what they are searching for for and spell it correctly.
“Discobot discoveries” is similarly out of place, but can at least be disabled.