That doesn't invalidate the point that at some point they thought "man, 6400 employees really enough to run Yahoo, we really need another 1600" so clearly they thought they felt had work for at least 6400 employees. So what was it?
A lot of not invented here (some of which has made it outside like Hadoop, npm being heavily inspired by the yahoo package manager, screwdriver which is a self-hosted travis clone).
Things that were once difficult but are now easy (datacenter management).
Things that are not user facing but which Yahoo is actually doing with much lower headcount than their competitors (ad tech).
Things which don't actually have synergies and have headcounts comparable to independent competitors (e.g. CNET is a 500 person company, Yahoo has a news business, Fanduel is a 2000 person fantasy sports company, Yahoo has one of those too)
