I’ve watching numerous documentaries but I actually never heard that we actually deciphered a message from within Earth!
If it is Atlantean or perhaps an older civilization, it is likely they are some of the vessels that have been spotted.
They could be doing scouting and testing(abductions) to better understand what has become of the surface world.
Possible they are the origin of Rh- bloodlines? They do share the same features as the giants, except the height.