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.
Worse still, how much of recorded history was subsequently lost?
Everything from the Library of Alexandria to lost episodes of Doctor Who. So much of what little WAS recorded is also gone.
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.
Thankfully we are having some luck in restoring lost content.
https://www.nationalgeographic.com/history/article/herculaneum-scrolls-mount-vesuvius-ai
https://www.theguardian.com/tv-and-radio/2026/mar/13/two-lost-doctor-who-episodes-from-the-1960s-found-and-regenerated