I hear this a lot. I know I'm far from the only one. Through my work I know how often Type I diabetics hear all about someone's Auntie whose (Type II) diabetes is controlled through diet alone. Hypermobility is another of those conditions where various forms of it exist and in this case the benign form is much more common. You won't believe how many times I've been told I should be good at gymnastics. Even the medical profession don't seem to always know the difference.
This NHS page is a good basic guide to Joint Hypermobility Syndrome and explains how it's way more than just being able to bend your thumb backwards (in fact, I can't do that), even if they do illustrate it with a picture of a thumb being bent backwards.
This page goes on to describe the diagnostic criteria - the Beighton Score and the Brighton Critera (I'm pretty sure they did that to confuse people). It was on that basis that I was diagnosed and the main issues I have now is with partial dislocations, bursitis and joint pain. The pelvis and hip joints are what are most disabling and why I can't manage hills or steps or walk long distances. And yet, I can still put my hands on the floor with my legs straight!