. Producing X-ray normally involves phenomena capable of splitting atoms, which lightning can't do (as far as we know).

Producing x-rays normally does not require splitting or transforming nuclei. The traditional way to make x-rays, say, in a dentists office, is just colliding high-energy electrons with a metal plate. It's mildly interesting to do the same in free air, but it doesn't seem to require any sort of new physics. Atmospheric electromagnetic fields are, trivially, strong enough to ionize a lot of air, and that gets you some pretty fast-moving electrons.