Frank's post relates to the fact that the upper 4 notes of a dominant 7th with a flat 9th make up a diminished 7th chord chord:
C7b9 = [C E G Bb Db]
Take the top 4 notes: [E G Bb Db] and you have an Edim7 and this can substitute for the altered dominant 9th chord with the root omitted (or as Frank puts it Edim7 substitutes for the C7b9 chord, whose root C is a major 3rd lower than the root E of Edim7).