Units can either build everything or not at all; what entities are in their build list is irrelevant. Although worker elephants can't initiate any foundation, they can finish all structures, twice as fast as any citizen.
This patch removes the worker elephant's ability to build and instead introduces an aura that gives nearby workers +20% build rate (i.e. reducing structure build time by a sixth).
As a consequence worker elephants are slightly less effective at game start but more efficient later when assisting larger numbers of units building large structures (e.g. centre, fortress, wonder).