Although they used a grounded wire, OSHA still says the cord has to be rated for hard or extra hard usage (1926.405(a)(2)(ii)(J)). Flat cords are not rated for this use and therefore are not allowable for extension cords in construction.
Reference Materials on electrical cords and safety:
OSHA Construction Electrical Safety & Health Page
OSHA General Industry Electrical Safety & Health Page
OSHA Letter of Interpretation: electrical tape on an extension cord
OSHA Letter of Interpretation: repair and use of extension cords
OSHA Letter of Interpretation: job made extension cords