The phrase David uses to describe GTD is "Mind Like Water," based on his many years of karate training. It refers to a perfect state of readiness, that will not over or under react. I do not recall David swappping in "Zen" interchangeably with Mind Like Water.
The Zen article on the GTD Times website is a community contribution, not authored by DavidCo staff.
I think he used to .. a long time ago .
For some odd reason I seem to recall David using the term in the old old Managing Actions and Projects Cassette Seminar that was published. [that was around 99-2000] before the book was even published .
I think he might have used it at a fast company seminar around the same time , also prior to the book .
I agree with what was said before , there's so much zen stuff out there anymore .....
Zen running, zen guitar, the tao of dating etc etc .
Hope that helps,