“εξ Αθηνών” is an ancient phrase meaning “από την Αθήνα” (from Athens).
In ancient Greek Αθήνα or Αθήνη or Αθήναι (plural) were variations of the same name. They referred to the ancient Greek goddess Athena and also to the city of Athens.
In modern Greek the name of the city is Αθήνα. “From Athens” is translated “Από την Αθήνα”.
The ancient phrase “εξ Αθηνών” uses the form “Αθήναι” (in genitive plural Αθηνών) and the ancient preposition “εξ”, which means “from” (από). This older phrase is rarely used nowadays and only in formal speech.
It is worth noting that in English the name Athens is plural, which corresponds to the ancient Greek name Αθήναι (also plural ).