Update:
I reiterated my request to DL again, got their final reply: "Respectfully, we are unable to offer the compensation you've requested as it's been determined by Air France's Legal team that EU compensation isn't due with regards to the delay of Delta Flight 49 to New York JFK on XX, 2015."
I am entirely unsure what AF legal team has to do with DL (except maybe that ours was a KL stock ticket, but operated by DL?)
Appears I now have to pursue this in courts?