I totally disagree. Delta has the opportunity to examine the supply chain to that flight and see if there was/is an issue a long the way. Lots of things could have happened.... perhaps a refrigerator is broken at the station where the food was stored, or perhaps there was some kind of mishandling.... could the meal have been left from a previous flight? They might not be able to find the cause, but then again, its possible that they they will, and this will help future pax.
I also agree with the poster who said to contact the health department of the city of departure. They track all complaints and it could trigger an inspection to be sure that best practices are being observed.

I am sure that Delta has very good regulations in place to prevent food borne illness, but this does not meant that these regulations are always followed. I've worked in food service....