You need to define the first question better. Who are people?
One random YouTuber? Your crazy uncle at Thanksgiving? The vast majority of humanity? Researchers specializing in memory?
On the face of it, I can’t see how it could be anything but a made up factoid someone pulled out of thin air simply because it’s too poorly defined, as others have pointed out.
If anyone is saying it, It doesn’t seem to be as popular as the 10% of brains thing but that could be that the 10% myth is trotted out by motivational speakers and life coaches more often.