Having spent 10 years flying sailplanes (gliders) as a hobby, I can tell you that this is not correct. A coordinated turn (and birds are much better at this than human pilots) at normal cruising speeds induces very little stress in the airframe.
Sure, it's possible to make hi-G turns that will snap the wings. Birds that encountered those limits wouldn't live to reproduce. Birds that grew to 200 pounds like your Teratornis were probably not doing snap rolls.
If your 50 pound limit were correct, then gravity would have to have been less that a quarter of what it is now.
... saying something like 'you're making us look stupid enough even at 40' since even that for a flesh and blood creature was impossible by any rational standards.
A 40-foot wingspan is about seven feet larger that a Cessna 150. The Cessna has about 10-pound wing loading at a maximum weight of 1600 pounds. The pterosaur you describe probably has a wing loading of less than 1 pound. The thing would float like a kite and have very little penetration (that is, it would accelerate very slowly and encounter very low-G forces in turns).
Lower the gravity to one fourth of the current value and the thing would need to carry rocks to be able to maneuver...
"It would be a grave understatement to say that, as a flying creature, Pteranodon was large. Indeed, there were sound reasons for believing that it was the largest animal that ever could become airborne. With each increase in size, and therefore also weight, a flying animal needs a concomitant increase in power (to beat the wings in a flapper and to hold and maneuver them in a glider), but power is supplied by muscles which themselves add still more weight to the structure.-- The larger a flyer becomes the disproportionately weightier it grows by the addition of its own power supply. There comes a point when the weight is just too great to permit the machine to remain airborne. Calculations bearing on size and power suggested that the maximum weight that a flying vertebrate can attain is about 50 lb.: Pteranodon and its slightly larger but lesser known Jordanian ally Titanopteryx were therefore thought to be the largest flying animals."