I have seen many lightning damages, and this is not it. For electrical shock to split a chimney without blasting bits of masonry all over the place is unusual. Further, the metal cap is still in place and intact, holding the top bricks in place, are we going to say the lightning was selective in which bricks to damage? Lightning follows paths of least resistance, which is why you may see a trench of soil created by a lightning blast that hits a satellite and follows the cable. This appears to be related to something internal, such as a piece of corroding steel, expanding and forcing the chimney apart. Did you ever get a look inside of the masonry?