Context: While the 90% figure is made-up, the premise is true in that most of human history has gone unrecorded. History is considered to have begun with the start of proto-writing (earliest being the Jiahu symbols in China circa 6600 BCE) to writing (3200 BCE), and thus the advent of recorded history.
Or is still in the ground. Less than 1% of all clay tablets have been recovered around the ancient Assyrians and Babylonians. There's an enormous amount of information that's still sitting there untouched due to politics, extremists, or lack of funding.