Phrases llike "he remembered everything" and "clearly remembered" can be warning signs when the research is being conducted by the wrong people. One of the aspects that is impressive here is the use of serious (even scholarly) methodology of checking, evaluating, collating, and comparing different sources if information. The participants seem to be quite sophisticated about what they are doing. Given dance's reputation as one of the more ephemeral of the performance arts, this seems a wonderful use of resources on the part of the Balanchine Foundation.
And, sometimes, wonderful coincidences occur:
Although Mr. Weiss clearly remembered the set and costumes, and a lot of Balanchine’s instructions, much remained unclear. Then, with the help of new technology, a miracle happened: a film of “Gaspard” from the Ravel festival, thought to be unusable because it was so dark, was digitized. Now a complete staging of the work seems feasible.
I am less confident that a reconstruction of Chant du Rossignol -- based on 70-year old memories of a dancer who was 14 at the time the ballet was made on her -- would be equally accurate.