The part you posted basically says some shudras were recruited to fight in Xerxes's battles in the (5th-4th century BC)...nothing more.
Please read the entire section, it talks about Indian influence, or Vedic influence on Persia, posting here:
From the book you posted:
Acc to this book, The origin of the varna in Persia is attributed to Zarathusthra,(7th century BC), and is pretty much a carbon copy of the Indian one,the RV version being far, far older (you mentioned 1200 BC), and rejects any persian influence in it, and actually implies the reverse.