If a cast bullet gives less friction than a jacketed bullet, doesn't that equal less pressure?
Yes, if less friction was the only difference in the two 'systems'. What's being suggested is that, while there is less friction, there is also generally a better seal due to cast lead upsetting into the bore more completely than a jacketed bullet can. I'm not arguing one way or the other about what does or does not happen, but the explanation does make sense, at least the way I understand it.
The one single factor that, IMO, makes it almost impossible to use 'common sense' or 'intuition' to explain actual results is that burn rate and pressure both depend on burn rate and pressure. Combine that with the liklihood that the
static friction of each bullet type--the amount of pressure it takes to get the bullet moving in the first place--is likely also different between the two, and so that impacts the peak chamber pressure, presumably.
I wouldn't expect a better-sealing bullet to make as much of a dent in the efficiency of a revolver though, in terms of percentage increase in MV. Sort of like trying to keep boat afloat that has a 2" hole and 1/4" hole in the hull, and you plug the 1/4" hole and expect things to improve. Again, intuition doesn't save the day here.
Anyway, rather than speculate and use intuition, this seems like a case where actual experience serves best. In an earlier post, identical loads with identical weight bullets from the same firearm produced what I think was at least a 10% increase in muzzle velocity. I know I generally end up using identical charges for both cast and jacketed, but have never intentionally done a side-by-side comparison. In tens of 1000s of rounds shot in handgun, I've never seen any significant issues with pushing cast just as hard as jacketed--the oft-cited threshold of 1,000 fps or so is pretty much pure nonsense.