I think that rather than pressure, what the statement is referring to is velocity. Pressure and velocity have some overlap, but different powders have different pressure curves which aren't necessarily related to the velocity of the projectile in the end.
Now what makes velocity important is how that affects the revolutions per minute (RPM's) of the projectile which in this case is a round ball.
That means that the greater the velocity of the projectile when it leaves the bore, the more that it will spin.
Every projectile needs to spin at a different rate to stabilize and to fly straight. Long, heavy bullets need to spin faster with more RPM's and velocity to be stabilized.
Short projectiles like round balls and light bullets require less spin to be stabilized in flight, therefore they fly straighter with less powder, velocity and RPM's.
But what throws a real monkey wrench into the works is the twist rate of the barrel. The faster twist barrel may impart too much spin to a round ball, especially if too much powder is used which results in too much velocity and RPM's.
That's why fast twist barrels are better for stabilizing heavier bullets. But by using less powder and slowing down the RPM's and velocity of the ball exiting the barrel, a fast twist can satisfactorily stabilize a round ball. Just don't use too much powder.
However with a slow twist barrel, more powder must be used to stabilize the ball by creating enough velocity and RPM's.

So the lesson of the statement is to not use too much powder or else the ball may be destabilized and spun too fast.
That's why heavy hunting loads with round balls may not be as accurate as light target loads.
Hunting loads are sometimes a compromise between accuracy and power. While target loads can be more accurate but lack some of the power needed for killing an animal.

This is all why it's important to incrementally work up a load starting with a lower powder charge and slowly increasing it in 5-10 grain increments until the accuracy is satisfactory and the groups are tight, which indicates that the right amount of velocity and spin is being imparted to the ball.

Of course the same method of working up a load also applies to testing bullets.
But with the benefit of experience, a shooter can estimate how much powder to begin testing loads with based on the twist rate of the barrel and the size, length and weight of the projectile.

More velocity equals more RPM's.
Less velocity equals less RPM's.

Balls require less RPM's to stabilze
Bullets require more RPM's to stabilize.

Slow twist means to use more powder.
Fast twist with a bullet means to use more powder
Fast twist with a ball means to use less powder.

This is due to how many RPM's is required to stabilize each shape of projectile based on it's length.
This is based on something called the Greenhill formula which helps to determine the best barrel twist rate for a projectile of a given length.
(Scroll down the page to twist rate - http://en.wikipedia.org/wiki/Rifling
Every size (caliber/diameter) ball has a favored range of barrel twists. But the favored range of twist rates can sometimes be fudged by controlling the velocity and RPM's.
Barrel length only affects velocity by a small amount of feet per second for every inch of additional length over a standard length barrel. So the amount that the barrel length will affect velocity and accuracy isn't usually too great, but none the less it's usually something that needs to be determine by range testing.
Welcome and good luck megabutter with applying any of these basic ballistic principles.