Extraordinarily courageous almost to the point of foolhardiness I would think. Perhaps he had a super able lab contact grading his rushes, or maybe his camera operator snuck a quick look at HIS meter. The usual DOP /Lab setup is to establish parameters via tests before shooting, so that rushes are single light graded to achieve quick turnaround (that's why they're 'rushes' in English language vernacular).
The financial costs of a single day's shooting usually far exceed the DOP's fee.
If the "sunny 16" rule had anything to do with it those rather large lighting setups usually found on motion picture sets, even on perfectly lit (for stills) days would be unnecessary.
Films need investors, not gamblers.