There are things that damage a camera when you point at them, but they aren’t passive. Things like x-ray sources could do that. Also the sun.

So no, even if you reflected 100% of the light from the flash back into the lens, there’s just not enough of it to do any damage.

If you were somehow able to focus all of it on one single pixel on the sensor, you might be able to damage that pixel, but that would require a large piece of optical equipment basically on top of the camera.