I hate it when people go "It's fantasy, why do you complain at all?"
The knight survived for several hundred years because of the Holy Grail. That's what the Holy Grail does. That's why everyone wants it. Either by magic, or by scifi, explain it any way you want.
But does a fridge really protect you from a nuclear blast? Would you survive the crash unscathed?
Had they introduced another magic or scifi device that protected him from the blast, I would have been okay with it. But a simple fridge is nothing that can protect you from such a thing.
It doesn't have anything to do with realism, it's all about logic.
I would have also been okay with them NOT showing how he survived. He gets in the fridge, closes the door, the nuke detonates, and FADE TO Indy waking up in the desert, with the fridge only barely visble as a HINT in the background. Or Indy waking up in a hospital bed.