When I signed up the sample they took was spit in a vial.
It never clicked for me that this was going to be the most scientifically accurate reading of my health in all the world. I took part because it could give me some scaring and soothing. But I assumed I'd go get a real test done if I was actually worried or interested about something specifically.
The website repeats this sentiment over and over, I learned about all kinds of interesting genetic stuff. I'm sure that's what it's for. I shouldn't be surprised that many people take what it says as gospel.