I have had the good fortune to read up on the Kalash people that the book refers to - without having the same good fortune in reading the book itself. But I can tell you that their pantheon is ( according to Witzel ) a quasi rigvedic one. Alexander does not feature greatly into this. Yama and Indra are much more prominent. The animist motifs are also a Hindu feature - like the rivers and mountains. You can compare that to widespread Hindu practice of holy rivers ( like Ganga, Yamuna , Godavari, Sarasvati, Narmada, Sindu and Kaveri - in a later shloka) and Kailasa the mountain abode of Shiva.

Where it concerns the broader South Asia, Alexander left little impact. His conquests in the subcontinent were limited to the frontier kingdoms and it is reflected in the Greek sources. We do not hear of a Greek army venturing beyond Indus - the heartland of the Arya India at the time - until Seleucus nicator who perhaps fails to attain any goals of his campaign and retreats and goes on to found Seleucid empire. I am sure people better acquainted with Alexander will attest to that. Thus it is not a custom that could exist in the heartland of the subcontinent and it doesn’t as far as I have read Indian history. Yavana ( Ionians possibly ) are decidedly outsiders in the works.

Ultimately, we are not required to go into ancient history here - since the book ( a work of fiction, mind you ) only deals with events in 19th century. That is the biggest red (herring? ) flag.

I think it is much more likely that the book itself was the progenitor of the tales. I have not been able to find any source in support of the hypothesis that did not ultimately refer to it.

Edit : clarification on Kalash religion