Can you download Atlas into ARK? if no, then it is not DLC
Can you find ARK in Atlas, well... let's see... Same dev's, same programmers, same code, same base engine, smart people do not start from scratch if they can avoid it, saves time, money, and work... could it have started as a idea for ARK? yup, could they have begun to make a DLC? yup, did they ? nope