An employer looking for a candidate has no reason to exclude someone with React experience though. If they want a candidate with Express.js experience, candidates may still have both. They just need the positive search in that case.
I completely disagree and you are focusing on a made up rhetorical example far too much. There is always benefit to more efficiently excluding results you don't want.
if you have a realistic use case please describe it.
Why? You are clearly missing the point and lack imagination. Another example won't satisfy you the way you think it will.