Most elite athletes wouldn’t be good at MMA or boxing.
Most people simply can’t tolerate punches to the face. Athletes are tougher than average, but most of them couldn’t either.
If every NBA or NFL player went into MMA, yes, a number would be championship level.
But there is no single player for whom you could say, he’d be championship level. The attributes required to be a top fighter are too rare.
There are plenty of young guys with good boxing skill, but can’t take a punch. Remember that the top fighters we see are the guys who survived this natural selection - they are the guys with abnormal chins. Even guys known for “glass” chins, probably have decent chins compared to the random male.
Can Lebron take a punch? Does he have a glass jaw? We don’t know.
If you gave me 100 different dudes with Lebron’s athleticism, I’d say a number would be champ level. But you can’t make that claim about Lebron specifically.