Alexander's conquered territory was almost identical to the extent of Achaemenid empire, ending at Indus. As such, it would seem that his primary objective was to conquer Achaemenid empire.
The claim that he was out to conquer the 'known world' (and related myths) was most likely later day inventions.
The phrase 'known world' might be a later romantic interpretation of the facts, but it doesn't change the basic facts.dont really understand such romanticism tbh, it makes a pretty logical sense how history turns out and yet it is twisted and turned in favour of one's biased narrative.
Alexander had access to the geography of Herodotus (5th century BCE) and of Aristotle (4th century). Both of those geographers thought that India was the eastern end of the Asian land mass. Aristotle said that from the peaks of the Hindu Kush one could see the eastern ocean. Neither knew much about India beyond the Indus. Neither geographer mentioned that India had a southern peninsula.