From the BBC article you linked to ...
The Harappan language died out and did not form the basis of other languages.
Dr Meadow: "The earliest inscriptions date back to 3500 BC."
"So probably we will never know what the symbols mean," Dr Meadow told BBC News Online from Harappa.
What historians know of the Harappan civilisation makes them unique. Their society did not like great differences between social classes or the display of wealth by rulers. They did not leave behind large monuments or rich graves.
They appear to be a peaceful people who displayed their art in smaller works of stone.
Their society seems to have petered out. Around 1900 BC Harappa and other urban centres started to decline as people left them to move east to what is now India and the Ganges.
So in this case, the civilisation lasted for less than 5000 years...
(Just for reference, here's the original quote you had an issue with: Every civilization with written records has existed for less than 5,000 years.)