GMAT Sentence Correction (SC) | Re: The Acoma and Hopi are probably the two oldest surviving Pueblo commun

As per your explanation "both" is an adverb modifying dating. And we know that COMMA + -ING modifier always modifies the entire previous clause. But I am not able to understand how "dating... " can modify the previous clause. I see no relation of -ING modifier to the verb of the previous clause. Can you please explain? Thanks a ton!!