Correct, Agema were recruited from Iranian nobility, it was first started by Alexander and later Seleucids continued this tradition. I guess they did that to buy support of noble houses while actually not giving them much since unit was small and not much more was done from military point of view. Alexander also used Iranian nobles for phalanx but as soon as he died, they got disbanded, leaving only Agema for Iranians. For their regular cataphracts it is also not clear if they were actually Greeks, they might have been mixed or even only Iranian.