Thank you.  I did not say it was a single book, but what you linked to says it is a 'corpus,' and it is often published at once, though apparently there is debate whether it was all written by Zoroaster.  At least for what Zoroaster wrote, I think philosophy does play a part, mostly involving semantics (difficult; knowing esoteric philosophy helps to interpret spiritually/divinely inspired texts) as you described, but perhaps also with gematria, or as HPB wrote, perhaps with 5 other methods than these latter 2.  Since many esoteric mystics have consensus on philosophy, not the least of which is ethics, methods of interpretation are important.  For example, such mystics would say animal sacrifice in the Avesta should not be done but is symbolic of something, as it is thought to be in the Vedas, etc..