I don’t understand why the author is surprised that the Sassanid had Arab Generals and Arab soldiers in their military. The Sassanid Empire ruled today’s Iraq, hence Arabs there. Yes, those Arabs were the very first to convert to Islam and there was betrayal - the Sassanid were primarily brought down from within.
Also, the population of Iran post initial Arab-Islam remained largely SUNNI for a 1000 yrs. There were Shia sects during that time but they were mostly Arabs.
It wasn’t until the Safavid dynasty (a Turkic speaking dynasty ruling Iran) starting in 14th century that Iranians were brutally forced to convert to Shi’ism by the Safavid, and had to say “Ali is Vali”, meaning “Ali is the guardian”. Shi’sm then became the official state religion in Iran, presumably to give Iran a distinct identity vis a vis others in the Islamic world. But it’s important to know that Shi’ism in Iran is relatively new.
Thank you for providing this background. Very helpful.