This just isn't true. Try listening to any deep ambient, and you'll be hearing (a) instruments with filter cutoffs in use (b) chords (c) no percussion (d) no lead. Again, Roach's "Mystic Chords and Sacred Spaces" is as good a place to start as any.
What is true in ambient is that you frequently do not want the third in the chords, because it makes the chords "too obvious". A lot of drone music involves a variety of chords with the third omitted.