It's a USB camera. It needs more time than a flicker to turn on and start producing frames. I don't think you could do as you said and still have the camera both work and the LED be dim.
The camera on Macs has actually been a PCIe peripherafor quite a few years now. But your point stands; it still takes a good second or so after the LED turns on for the camera driver to start producing frames to userspace.
