No, it's not at all myth....the "Anciend Indian Civilization" can be well defined...because there are enough historical evidences.....Therefore it is also acknowledged by the West.
Generally speaking, Ancient Indian Civilization(Indus Valley Civilization and the civilization along the river Ganges) spread from modern day Afghanistan in the west(the existance of Bamiyan Buddha status-destroyed by the Talibans and HinduKush mountains proove it) to modern day Burma(Myanmar-Chola dynasty stretched from South India to modern day Myanmar) in the east.......
The Indus Valley Civilization flourished around the Indus River basin, So it is no wonder that most of the remains of this huge civilization falls in modern day Pakistan.
BUT, The Indus Valley Civilization which was a Bronze Age civilization flourished during 3300–1300 B.C.
And, Muslim rule in the subcontinent began in 712 A.D.
So there was no Islamic influence in this civilization.
The name “Hindu” was given by outsiders, invaders(Persians) who could not pronounce the name of the River Sindhu properly.
Therefore people living at Sindhu river basin and beyond came to be known as "Hindus" and hence Hindustan.
NOW, people living at and beyond Sindhu river basin(towards further East and South) actually followed the Vedic Religion.
So, the followers of this Vedic Religion, came to be known as "Hindus".
Therefore what is known as Hinduism today, is actually this Vedic Religion.
Though people claim that this vast landmass was not unified under a single rular, there were many warring dynasties, so there was no ancient unified INDIA as such.....BUT what these people forget(may be intentionally) is that, even though there were many warring dynasties, they were still UNIFIED by one single binding force...HINDUISM(vedic Religion)
So, India(or Hindustan or Bharat or whatever you call)....was very much Unified and a single entity.