good theory, but no.
he got better milege driving exactly the same he did before hand.
he made his kit, and changed nothing about his driving style.
short trips, his milege improved 50% above stock... so 100km stock = 150km HHO
long trips, milege dropped 50% below stock, so 100km stock = 50km HHO
your theory works for you and others that change their driving habits, but considering you don't know this guy, how he drove pre/post, or anything other than what little I told you, your "assumption" is just that.
considering he drove very little, until those long trips back home, he removed the kit after 1 month. he re-set his 'CEL' each time, and would un-plug the kit for long trips each weekend. he decided it wasn't worth the trouble before each road trip to un-plug it, just for an increase in milege over short distances.
anyway, good thought, but wrong application.