He might have even gotten a coach seat deliberately to avoid running into Dimitrios. Since Bond secured a seat on the flight at the last minute, he may have had to get a coach seat against his will. If this flight was a small aircraft, it might not even have a first class (a Delta CRJ200, for example, does not have a formal first class area - just 'economy comfort' seats and JetBlue doesn't have first class on any of its flights). There are several possible explanations for this.

Regardless, it's extremely unlikely Dimitrios would not have spotted Bond - who had just robbed him of his beloved Aston Martin DB5 only hours earlier. It would be a relatively small aircraft flying the short distance from the Bahamas to Miami and let's face facts, both Bond and Dimitrios would probably be flying first class. Bond then unceremoniously dumps her, leaves the beach condo and catches the same flight. While Bond is pumping Solange for information, she receives a phone call from Alex Dimitrios saying he will be on the last flight from Nassau to Miami.

Casino royale hotel scene