While these cables are dumb, they aren't completely brainless. Each USB-A-to-USB-C cable is supposed to have a resistor inside that limits the amount of power that, say, a fast-charging USB-C phone can draw. That's important because most of today's USB ports were designed before USB-C came along.
While some full-size USB ports, particularly those on phone chargers, can output 2, even 2.4 amps, there's no guarantee that any old USB port will provide even a full amp of power. But a fast-charging USB-C phone can draw 3 amps. So when you connect your USB-C-equipped Nexus 6P phone to a MacBook Air port designed to only provide 1.1 amps at most and use a cable without the right resistor, the phone can pull too much power, create a voltage drop inside the computer, and potentially destroy internal components.
That's what probably happened to Dieter Bohn, who connected those exact two devices together with one of the very first cables that Leung had warned against: an Orzly cable that used a 10k ohm resistor instead of the 56k ohm resistor that the USB spec demands. (These resistors don't actually meter the flow of electricity, they just tell the phone how much energy is safe to pull. Orzly has already revised their cables to use the right resistor.)
Browsing through Leung's Amazon reviews, the wrong resistor is far and away the most common reason a cable fails. But you also don't see a lot of people complaining about toasted electronics.