The “versions of MacBook laptops and iMac desktops” they are referring to are the ones from more that 8 years ago that didn’t have the hard wired LED. Nowadays there is literally a wire from the sensor to the LED. If the sensor is on, the LED is on. It’s got nothing to do with firmware.

What happens to the connection if the LED burns out? Is the LED lifetime sensitive to state change such that you could flicker it via rapid camera toggling to burn duty cycle overnight?

If someone had access briefly to your laptop at a coffee shop, could they cover the LED with black opaque cellophane?

This sort of attack is pretty specific, so you figure it’s only done by people who want something from you in particular. These sorts of physical prep steps are what I’d be looking at if someone were relying on the light. Hypothetically, I’d attack the light first then take advantage of the false assurance.

Missing or open cover is a little harder to miss.

LEDs don't burn out in regular usage conditions

I can’t say if that’s still the case but that study is seven years old.