USS Thresher was lost through a chain of system failures

Was USS Thresher lost because of one badly made pipe joint?

No. A probable joint failure triggered a cascade involving design, construction, inspection, maintenance, flooding response, and propulsion, leading to system-wide safety reform.

A failed seawater connection may have initiated flooding, but it was only one link in the loss. Inspection and quality-control practices had allowed vulnerabilities to remain in a submarine operating near its limits. Water and ice affected electrical and compressed-air systems while the reactor and propulsion response left the boat unable to regain depth control in time. The resulting SUBSAFE reforms treated the accident as a system-wide engineering and assurance failure rather than a single defective component.