how do we know louis cyr didnt just make up shit either?
Honestly, we don't. But at least it is closer to a real believable number. Cyr's was done at an exhibition with 18 large men standing on a platform. I'm not sure if they were weighed or not, but it was probably close to claimed weight.
I think the popularity of the back lift came from circus's, fairs, and various stage shows, etc...... They'd load up these things with audience members, horses, elephants, showgirls, etc..... and it would be a treat for the eye. When that type of entertainment faded away, you hardly ever saw the lift again.