Whole point they're trying to make is that while the PSU does move some air in and out of the case, it's minimal. The CPU cooler only circulates air around -inside- the case, and without something to put fresh air in (and thus out through the other side) it'd be like covering yourself with a blanket. Eventually the air gets stale and hot and you're gasping for air.
If the case comes with a fan, hook it up. If it has two, have one blowing into the case, the other blowing out of the case - this creates your flow pattern.