I agree with the British Indologists on that, not because of any Muslim assessment, but because I follow the empirical method and empirically we do not have any extant documents on Buddha's life which are that old. The extant copies of the Puranas date from between c. 400 BC and 1400 AD, and documents written after 400 BC are not and cannot be eyewitness sources on events from 3000 BC. Empirically we would need some sort of pre-Puranic evidence to support any traditions about Buddha being from 3000 BC. I am not aware of any such evidence. I am aware of the pre-Mohenjo-Daro find mentioned elsewhere in the thread, from previous discussions of this, but I would consider that evidence of early civilization in India rather than evidence of Buddha living c. 3000 BC.
I won't be too concerned with the dating of Sanskrit documents, just that classical Sanskrit was frozen in form at a particular date and that it is something that can be learned and read. English speakers already know Sanskrit, of course, it's just a matter of sorting which pieces of the language are Sanskrit and which are not since it is all jumbled together.
According the Puranas, Buddha's birth was predicted - to happen later. So there is no contradiction, rather there is agreement in the timing of his birth to be around 2600 years ago (IIRC).
The Puranas were an oral tradition before they were set in writing, and since the system in India was to write on palm leaves, which had a rather short life span, the four Vedas and their corollaries such as the Upanishads, Puranas and so on were copied and recopied continually, sometimes with new commentaries or additional material added. That is probably why some historians consider them of more recent origan; that and the fact that historians generally have a set idea of history and like things to fit into their theories rather than be open to different time scales.