To steelman the argument, maybe they meant "the monitoring system is good enough at catching defects that if none are reported, the engine will probably pass an all-up hotfire test of the engine".
Of course you could do a water or air-pressure leak test on the plumbing pretty easily, and you would likely do that on the first 30 engines...
But if you have confidence in your build process, maybe the juice isn't worth the squeeze on (say) a direct contact ultrasound void check on every square millimeter of the part.
It's all about "how expensive is it to run the test" vs "what is the likelihood the test catches an issue" vs "what's the cost of failing while everyone is watching?"
Same reason SpaceX went from dry-dress-rehersals to wet-dress-rehearsals to separate-static-fire-before-launch to hold-down-for-three-seconds-before-launch... The hold-down -before-launch is an integration test that covers everything the previous tests do, so eventually you can start removing redundant tests.
Of course you could do a water or air-pressure leak test on the plumbing pretty easily, and you would likely do that on the first 30 engines...
But if you have confidence in your build process, maybe the juice isn't worth the squeeze on (say) a direct contact ultrasound void check on every square millimeter of the part.
It's all about "how expensive is it to run the test" vs "what is the likelihood the test catches an issue" vs "what's the cost of failing while everyone is watching?"
Same reason SpaceX went from dry-dress-rehersals to wet-dress-rehearsals to separate-static-fire-before-launch to hold-down-for-three-seconds-before-launch... The hold-down -before-launch is an integration test that covers everything the previous tests do, so eventually you can start removing redundant tests.