really, it is true.
in my mid 50's, training for a marathon (that I ran in 3:15-3:20 range- trying to be anonymous here)- I alternated walking and running days and the walking was in the low 10's, 5-6 miles per day (on days that I walked).
Walking is like any other sport or event- it seems impossible when you first start, but if you try to walk fast, you'll get a lot faster pretty quickly. Most people just don't want to do this, so they don't try, for a variety of reasons. The walking we have been doing all our lives is not at all specific to walking with a cadence of 170-ish. The main point: yes, a runner can learn to walk really fast and get a great aerobic stimulus.
On the other hand, anybody who can walk that fast most likely is no longer injured and can resume running- but it can be a good form of cross training.
To be clear- I'm not talking about racewalking technique, and the straight leg that is required.