The imperial creed wasent even started by the Emperor.The Emperor disliked the concept of worrshiping him as a good.during the great crusade the emperor went as far as destroying the Word bearers' home planet because the Word bearers thought that the emperor should be worrshipped as a god.
and to simplify this:The Etherials are atheists (meaning beliving in no god) and the emperor is also an atheist,which is why before the great crusade he demolished all religious centers.
The Etherials are also believed to have psychic abilites to keep other Tau under control.Just as the Emperor did with the Imperium of man.
The Emperor was born not only to protect mankind but to protect the galaxy from chaos,Some canon sources claim that the Emperor was a collective of the most powerful human Shamans during very ancient times (8,000 BC) these shamans created the Emperor in order to battle chaos since the Shamans were weakining from fighting chaos.
The Etherials do not allow the Tau to make research on the Warp and psychic ablities.This is due to the Etherials wanting to protect the Tau from chaos taint.
and therefore it would not be ridiculous to think the Etherials could be agents of the emperor.