16,500 PSI was just what it was tested at, and it could apparently withstand significantly greater pressure than that. From the Deep Sea Challenge website:
"The hull, complete with its hatch and viewport, was tested twice in a pressure chamber at Pennsylvania State University to an equivalent full-ocean-depth pressure of 16,500 pounds per square inch (1,138 bars). It passed both tests. Twenty-two strain gauges attached to the sphere gave data that indicated the sphere could withstand up to 140 percent of the test pressure without buckling."
"The hull, complete with its hatch and viewport, was tested twice in a pressure chamber at Pennsylvania State University to an equivalent full-ocean-depth pressure of 16,500 pounds per square inch (1,138 bars). It passed both tests. Twenty-two strain gauges attached to the sphere gave data that indicated the sphere could withstand up to 140 percent of the test pressure without buckling."