why did hitler use the term "Aryan" a name that referred to ancient Indo-Europeans to describe modern Germanic peoples?

He believed in Atlantis.

Explanation:

Hitler was in a sort of cult that semi-believed in Atlantis, the Lost City that sank under the waters. They believed it to be somewhere in the Mediterranean and was populated by people known as Aryans. Obviously, the Aryans must have been physically and mentally superior to create mankind's most advanced city. In addition, some texts they had corroborated with this idea, so Hitler just applied this to the German people.