Having been a professional pilot for nearly 30 years, I can state these as facts:
1. Hail can indeed do this type of damage. Unlikely in this case because it would also have damaged a large part of the skin of the aircraft, especially the leading edge of the wings.
2. 18,000’ is not the highest altitude where hail damage has occurred. Hail is created by water being constantly “rolled” up and down inside of a thunderstorm cell by up and down drafts. The level of energy in the cell determines how long the water “rolls” and when finally released, that alone determines the size of the hail stones. You have seen “golfball” and “softball” size hail in the news. The larger ones usually occur in the more southern states, where thunderstorm cells can reach in excess of 60,000’ altitude.
I remember flying from Sydney to Hono, and over the middle of the Pacific seeing massive thunderheads, as if we were on the ground.
Really impressed me, since the 747 was probably at 37,000...
If you slice a hailstone at the equator it looks like a tree ring slice.