In my experience as well as what I've read, there's some difference between cramps I get during exertion and the ones I get afterward.

During exertion, the cramps I get can be any number of things ... the more unaccustomed the effort, the more likely it is to fit Schwellnus' theory. IIRC, his study examined only people who cramped during their Ironman races. Along those lines, it's pretty common for me to get foot cramps late in a long race, though I almost never get them on a long run at an easier pace.

The ones I get later on, and most particularly the leg cramps that wake me up at night - I can treat them pretty effectively with a quick-absorbing calcium-magnesium supplement, and I can mostly prevent them as long as I titrate my maintenance calcium-magnesium supplement along with the amount I sweat on any particular day.

I'm pretty prone to a mild hyponatremia, but AFAIK I've never had cramps related to sodium levels. YMMV.