I think what they're saying is that while East Coast flavor of swing is phrased in 6 counts to 4-beat music (so that 1 pattern takes up 1.5 bars of music), so you start the pattern on the strong beat 50% of the time. The other flavors of swing are 8-count, so they do fall on the phrases better, but of course are also complicateder, and can throw in 6-count patterns as well just to mix things up.

I personally think that a lot of proper swing music is phrased to accomodate that, although what gets played at a lot of swing dances is not quite swing music. (I _can_ swing to rock-n-roll, it just doesn't make me _want_ to the way big band does).