I would imagine that mountain lions are pretty tough.
At close range, nothing is more lethal than birdshot. Ask any Paramedic, like me.
I worked 14 close range birdshot to the torso calls, every one but one was DRT.
The last one died the next day.
This was inside the house range, 10 to 20 feet.
Ten yards would be pushing it, but it does not surprise me that it killed the lion.
Sounds like the lion wasn't charging the guy, I can't say what I would have done in his situation.