So you are wanting a user:johnsmith search to search all posts in topics created by johnsmith even if he didn’t post them?
Yeah, or more generally, if I searched for:
user:johnsmith keywords
I would expect to see results that could be described as “topics involving johnsmith and keywords.” At least that’s sort of my intuitive expectation from using major search engines, and it seems to be important at least for technical Q&A search.
So maybe topics he started where the keyword is mentioned, but perhaps also just topics he posted in where he or someone else mentioned the keyword.