It is the distance work after the Intervals that is the "fartlek", being just a portion of the overall workout as Bowerman seems to have designed it.
http://www.letsrun.com/forum/flat_read.php?thread=432770&page=0
Scroll down to KnutK:
"He was as mentioned a believer in fartlek running. These are often described as easy runs. Bill was very aware and in many ways encouraged some of us to run some of these runs in a solid pace. These were runs between two sets of interval work in one session. Some of us we put in a major part of our training effort on these runs"
Or in Kenny's example, the "five-mile run through the hills" that is the "fartlek":
http://www.kennymoore.us/kcmarticles/The%20Man.pdf
"I was allowed our regular hard-day sessions of four three-quarter-mile repetitions at 67-second-per-lap pace, then a five-mile run through the hills and rhododendrons of nearby Hendricks Park and four fast 110-yard strides before showering."
Perhaps Bowerman's definition was simply different hence the original question, and fartlek for him simply meant the running portion of the workout away from the track, whether loosely planned or spontaneous.