There is an optics law that says that you can not concentrate light to a higher concentration than it's source. If the moon was the source then the light could not be concentrated stronger than it is on the lunar surface. I frankly do not know if it is strong enough there or not, but I highly doubt it.

Since the sun is almost equal distance from the earth and the moon and the earth has 1/3 to 2/3 of the sunlight filtered out by the atmosphere, I am willing to say that the moon's surface has a brightness of 3 times high noon here on earth. Unfortunately that brightness is radiated or rather reflected radially in a spherical pattern and we are 1/4 of a million miles from it.

Because of the way it is reflected and diffused, not lensed, the light can effectively be considered to be generated at that intensity at the moon's surface. Following that assumption, and applying the law that the lensatically concentrated intensity can never exceed that of the source, the concentrated moonlight can never exceed the intensity of earth normal high noon, which I have never seen start a fire without further concentration which is not theoretically possible in this case.

But what do I know....