[antman]

There was an unprecedented failure of correlation vs causation and it was based on crocodiles. The whole idea was that the Indian Ocean was a lake and they would circumvent it, reach the top of the Nile and then sail to down to return to the Greek territory. Unfortunately this geographical mistake was based on the assumption that crocodiles (and lotus plants) they saw in India existed only in upper Egypt only so the two regions were somehow connected.

Regarding Alexanders army, most of it was disbanded after he took the 4 main persian cities and then it was mostly local former persian empire subjects and officers (heteroi - εταιροι) with Macedonian Generals. They probably had a fair knowledge of the situation. The Persians said that they had already occupied some part of Indian territory but there are no Archeological evidence of sustained Persian presence after the Khyber Pass.