John,
You're obviously too bright to be this dense so I have to assume you are being deliberately obtuse.
I accept the fact that the hill was in cloud but, as other people here have referenced, there seems to be certain situations where a cloud that obscures a hillside blends in with the distant sky such that it appears to be open sky ahead.
" In light winds and high relative humidity, layers of fog and low cloud form adjacent to the Mull to obscure the high ground completely. There are occasions when a relatively calm sea and a grey sky of similar hue merge, so that in otherwise good visibility a pilot flying in accordance with Visual Flight Rules may be unaware that he is about to enter a fog/cloud bank."
Link
My emphasis...
There are issues which the Board were unable to totally discount (though apparently the Air Marshalls could!)
They were:
a. Spatial disorientation or visual illusion
Link
I hope that helps you visualize the scenario and allows you no other way to be evasive...
