two things about todays 100 LL, it is a lot thinner, less specific gravity, than todays pump gas or todays race gas. Maybe he was referring to this As far as altitude and the design of Av Gas back in the day when commercial pressurized airliners ran Av gas they did tend to fly a little higher on the altitude than your normal non turbo charged or super charge piston engine airplanes fly normally for several reasons.
Most piston driven non pressurized airplanes used today fly at or under 12,500 ft. above sea level for several reasons, one of them being the pilot is required to use oxygen above 12,500 if they are above that altitude for more than 20 minutes.
I flew a friend and his grown 30 yr old Son to Denver and back from SO CA around 20 yrs ago, I had a five place oxygen system in the airplane with new canula for breathing at or above 12,500 ft. I had to go above 12,500 ft. going and coming back and they didn't want to use them, they both went to sleep and where groggy with a head ache on the return trip. I decided to fly direct from Denver to the Grand Canyon airport on the return trip to save time and fuel as we where able to depart the small airport in south Denver before 9:30 AM. I flew at 16,500 Ft for around 1 hour before descending down to 8,500 near Gallup N.M. so they wouldn't get oxygen starvation and suffer for the rest of the day. That didn't work as the damage had already been done
I had to lean the mixture out mechanically from the cockpit with the leaning mixture knob anytime I flew 3,000 above sea level to avoid fouling the spark plugs I accidently foul one spark plug on my second flight after buying he airplane, only once That is not a good feeling when you loose the use of one cylinder on a six cylinder engine and your 3000 + Ft above the ground