3 hours of racing in a car pulling 0.9-1G is demanding, sure. But not even on the same level.
Ground effect cars were very quick in corners for their age, but we passed that point in the early 00's - on grooved slicks, no less. They peaked at approximately 4G in sustained cornering whereas current cars are knocking on 6G.
To give you an idea of the demands that places on a driver, consider that fighter pilots are only expected to be able to sustain that sort of force (Vertically, admittedly) for a few minutes. F1 drivers sustain that for an hour. To compare it to scooting around in a Type 49 for a few hours is frankly a joke.
The reason drivers were exhausted, collapsing in their cars and blacking out was not because the cars back then were more physically demanding to drive, it's because nobody trained as intensively or as effectively. Even those drivers who weren't drinking themselves to death and smoking 10 packets of Gauloises a day didn't train as hard as current drivers, and certainly not in a way that would prepare them for the significant G-force they were to face. Think - if people were blacking out and collapsing in cars pulling 2-3G, imagine what would happen in a car pulling literally double that amount.
If you want to move the goalposts and talk about racecraft, ability to drive older cars etc, then I'm more than willing to agree on the genius of some of the older drivers, but the point you took issue with still stands - Those drivers would not be quicker in the current machinery even when they were at their best.
It's the same rose tinted view that believes that Group B was for men and everything after is rubbish.