Simply put, it is unlikely that Zurvanism ever existed as a separate practice distinct from Mazda-worship (the primary endonym of Zoroastrianism). The phrase "Zurwan Akarana", or "unbounded [span of] time" which occurs once in the Late Avestan Videvdad, seems to mirror the Greek term "Chronos Apeiros", and the handful of times Zurwan is mentioned it is alongside other highly abstract divinities, e.g. "Space". In another mention, the power of Zurwan seems to simply stand for an expression of fatalism.

The particular story of the imperfect Zurwan creating Ahriman and Ohrmazd as twins known from Christian sources may simply be one of several Zoroastrian "meta-Cosmogonies", elaborating on a point where Old Avestan sources are ambiguous or silent (viz., the relationship between Ahura Mazda, Spenta Mainyu and Angra Mainyu), possibly coloured by stereotypes about "Gnosticism" or Manichaeism.

Boyce and some following her reasoned that this was the "Official" cosmogony of the Sasanian court at one point or another based on its appearance in Armenian Christian polemics presumed to argue against Sasanian "official" narratives. Boyce in particular made rather far-fetched connections to pre-Achaemenid Median religious practices for reasons that honestly never made much sense to me.

Likely, the precise degree of influence any form of "Zurwanism" ever had will never be known. Regardless, Zurwan does not appear to have ever been an important target of worship, and therefore the posed dichotomy between Zurwanism and Mazdaism is misguided.