Good question! I read two different Amazon press releases on this but still had to come here for the answer. It seems strange they don't want to advertise the ISA of a compute product - does marketing think it might scare people away?
At this point I think they just assume that everyone who cares already know that graviton=arm
It seems they don't document the ISA for any instance types. This could be deliberate (and unrelated to marketing) in case they decide to pull features from the instance types in a microcode update. Without any ISA specifics, previous customer commitments towards instance types would still apply.