Hi All -
What they may mean is that because a jib is set on a wire and has a clean leading edge it has the ability to point higher without stalling ,--The mast -typical section --addes a disrupt factor to the leading edge of about 10 to 15 percent dependant on wind velocity ,--according to what I,ve read ,-
The advantage of adding a jib other than more sail area is the attached flow aspect ,-particularly in seas when the rig is being swung around making it more difficult to keep attached airflow over the sailplan ,--We have all seen out tell tails in light air and choppy seas bounceing all over .

The lead of the jib deflects the wind approaching it ,-so by the time it reaches the main it is deflected at a slightly higher angle which requires you to sail a little lower to keep the same apparent wind angle ---Thats why a main only rig sails higher ,but has a more difficult time keeping attached flow in seas and tacking .

-Think of the apparent wind with the spin up and how much that effects mainsail airflow ,--its generally centered on a cat downwind ,--The jib does the same ,just proportionately to the size , and in this discussion the upwind mode.
If you can look up wind tunnel tests of rigs that use colored smoke ,--it makes this very visual --
Hope this perspective and explaination is helpfull ,
Always enjoy design related discussion posts ,-thanks
Carl