Perhaps they had you confused with the later Mia-GCM flt tonight which cxld?

Or maybe they were told they were going to cxl all MIA to caribbean destination flts today but then found they could get some out before it hits?

Or perhaps he was looking at the GCM-MIA return that is also flt 378 that was supposed to come back. They very well could have misread their flifo screen in error having saw flt 378 was cxld not realizing it was the return part only.

The last one would be my bet. Unfortunately not all employees are good at fully understanding the flight numbers. Sadly too many agents don't realize same flt number does not mean same plane, in fact a lot it's not the same plane.