The river itself dried in around ~5000BC.
The text attributed itself as being authored in 3000BC.
The idea that text was written in 1500BC (3500 years ago) was proposed by Max Muller which was contested by his contemporary western and Indian scholars alike ( they proposed earlier dates)
In his late life, Max himself admitted that the dates he gave were hypothetical, they were to considered minimum and it could actually be more than that.