A friend of ours who booked on Delta out of Newark for February had her flight canceled. My wife and I booked on Jet Blue in June out of Newark nonstop for January, returning end of March. Recieved a notice that our non stop now went Newark to Boston to SXM and reversed on the return trip. We then received another notice in September that our return flight would go from SXM to Boston with a 20 hour layover and then fly to Newark. I canceled Jet Blue for a full refund and booked American out of Philadelphia non stop to SXM. It was double the price of Jet Blue but less agrevation. Seems the airlines are all having staffing issues causing these problems.
It appears that the email you got from DL is or was wrong. We have sources in most of the airlines and our DL source says these flight segments are available for booking on the travel date you published: MYR-ATL 0600-724 ATL-SXM 0945-1445
You might want to contact DL again and ask for a customer service supervisor.