I agree with everything you say until the last paragraph. My perception of Reich and Glass as composers is that despite coming from the same city, during the same period, and being lumped together by history they feel like two composers whose style and methods run parallel but never quite meet. They've both built reputations around styles reliant on repetition, but the ways they employ it and the character of their music are very different.

While the musical motif of Reich's 'Piano Phase' - an ostinato arpgeggio - is something that Glass also employs in his own music, their individual uses of such a fundamental and common musical figure diverges tremendously. Reich (for the most part) fixates on that single motif and the patterns that appear by "phasing" it in and out with it's copy. Glass' typical approach for this period is to develop the figure through harmonic development or through the contraction or expansion of the phrase's length.

While you could argue that Glass' style developed from Reich's initial idea. To suggest that Glass' entire catalog of the period is somehow fully contained and found in the content of 'Piano Phase' and never added anything new is a major stretch.

That would be like arguing that everything Led Zeppelin wrote was already implicit in 'Johnny B. Goode'.