But you surely have to factor-in the context here.
Stone & Gibson weren't observing rockets on a warm, calm safe night in the middle of the Caribbean. Had they been, they would've understandably assumed they were company signals & barely raised an eyebrow.
Instead, these rockets were emanating from an area they knew was dangerous. In those circumstances, rockets of any kind would surely be at least some cause for concern.
Ask yourself how plausible this sounds:
"Look, a vessel on the edge of that dangerous icefield is firing rockets"
"No, those rockets don't conform to the official Rules Regarding Distress Rockets"
"Oh okay, no need to worry then. Fancy another cuppa?"
Sorry to sound flippant, but this all seems incredibly obvious to me (albeit I lack the requisite two-hundred years nautical experience to know what I'm talking about).