Maybe he had a Star of David necklace, but I found it pretty unusual. How was the faith of unknown soldiers determined?
In the book 'The Last of the Doughboys' by Richard Rubin, he mentions that General Pershing was the head of the American Battle Monuments Commission from 1923-45. In the book, it is mentioned that Pershing instituted a policy where a percentage of unknown soldiers, in accordance to Jewish percentage in the general population and army, should be given Jewish tombstones.
It is possible this is why this WW1 unknown soldier has a Jewish Tombstone.
PS: I don't have the book with me on hand to find the page number etc. But I read it a few months back.