Here we go again. It's like teaching my Isa Browns algebra.
If you take off on a clear night out over the water. SKC. You look down and ahead and see nothing. Are you suddenly now IMC?
Yes, less of the mountain is at 6000 than 1500. No argument there. All other things being equal, the impact at 6000' would have been about one minute flying time further south. There would have been few if any extra clues to stop them. The MSA is another 10000' above, which is where they should have been.