My take is that while ask.discourse.com cannot answer the same search questions because it does not have access to the community’s content, it does provide a useful example of the framework—both in the UI and, presumably, the underlying implementation—for supporting a much larger prompt input.
Of course, something like leaves us hanging a bit on what that might eventually mean. It feels like waiting for the first episode of the next season of a show. 
Now that Discourse search, when configured with Ask AI, can handle questions of this kind, I would rather see the search interface grow to accommodate that capability than have it unnecessarily constrained.
In all honesty, I have tended to avoid Discourse search because I often found it more frustrating than useful. With Ask AI, though, it is starting to show real promise, and I would hate to see the input UI limit what the feature itself can do.
In a way, this reminds me of the challenge Google faced in evolving a search interface designed around a few keywords into something that also needs to accommodate conversational AI queries. 