Which is pretty evident if you’ve ever spent time in typical commercial flat-roofed buildings. Who hasn’t seen buckets catching drips at some point or other?

In my experience that is true until you go with a foam roof. I've seen two commercial buildings now that used an expanding foam sealant/roof which creates about a 6" seal on the roof which you don't normally walk on (on both that I saw wood pathways were laid down between ceiling access and the roof machinery (HVAC, skylights, etc). One of them had the roof installed in 2001 and has not had a single leak according to the landlord. (I know, biased, but he was trying to sell me on the building so if he was taking a risk if he was lying)

Apparently it "naturally" bulges a bit in the middle when they install it so the water is shunted to the edges. And of course California doesn't have extreme rain events all that often.

I was thinking the same thing, despite the article saying flat roofs were fine as long as they were reinforced against load. The load is rarely the issue, but as you stated, flat roofs seem to ALWAYS leak.