I am with A Squared on that - I believe it would have been off.
These aircraft fly IFR enroute, and once arriving at the fire ground operate exclusively VFR until picking up an IFR clearance again on returning to the aerodrome.
I fear that in the pursuit for a successful retardant drop, the crew might have inadvertently blurred the lines of what 'visual' meant. Perhaps this is an absolutely extreme case of 'get-there-itis' but in a far different circumstance where the result of aborting the operation is far different.