Yam speed buff applies before Deer stones is completed

This scout has the Yam speed buff before age 2. Opponent was building deer stones, but it was not completed and the scout ran near an outpost and was granted the buff.

I think cavalry and traders get yam speed buff from outposts inherently. Deer stones gives it in an area effect to all units and provides the castle age tech for free that gives it to all units near outposts too.