Ok, having done a bit of flying in Africa, and other dusty locations with the 139, let me just say that the "Company's (Caverton) Brownout Landing Procedure" is unlike anything I have ever heard of. Cat A gate of 100/20 is also counter-productive to setting up for a brownout risk landing. Can't fault the pilots, they were following their company procedure. This is one of the consequences when all your work has been airport runway to offshore platform and back, and your procedures reflect what the oil company auditors have rewarded you for. Immediate Safety Recommendation 2019-003 would indicate a simple "never go" since any risk of brownout is going to blow the analysis off scale. And frankly, if that is the brownout procedure published, it is just as well that everyone stays home.
Anybody think to ask the police 412 what his secret was? Those guys probably do a lot more routine dusty landings.