the proof of liquor is double the alcohol percentage, so if the scotch was 150 proof, it is indeed 75% alcohol. However that percentage is highly unlikely. I drink 18 year old Glenfiddich which is 86 proof or 43% alcohol.

If it was aged 20 years and then bottled and it was around 150 proof or 75% alcohol it would have been that when he drunk it as it doesn’t age once it’s been bottled.