I'm guessing too that they meant they may not supplement as high a percentage of their diet from free ranging as perhaps some other breeds that tend to wander farther afield. Perhaps their laid back personality might also put them at a higher risk to predation than some of the more nervous breeds.
We had a hen adopt our place a few years back, after the neighboring farm sold. She was out of working game lines. a completely different animal than my Brahmas. Ranged much farther, and was much more aware of everything going on. She was an excellent broody, saw her kill a garter snake once when she had chicks.