Surely if given people vitamin D can’t actually cure MS. Surely someone would have tried it and done case studies?

It's not a cure, it's a very effective therapy. It has ~10 years of history and trials. The doctors I've spoken with mention interesting new studies and research associated with the therapy often, so it's still a developing area of research... research that has moved past the "...in mice" caveat.

Having read the page, this is particularly high doses, along with a clinical protocol to determine the both correct dose, and other treatments required to avoid serious problems that would normally accompany such a high dose.

Simply supplementing with D3 would run into problems without that. Nevertheless, they do cite studies showing improvements with lower doses.

Normally I'm pretty sceptical about 'named doctor' websites as they are usually some kind of grift, but this one doesn't seem to be.