That airplane snitched on that crew when this incident happened via FOQA datalinks. The 777 is equipped with a real time data modem via Inmarsat that sends back to the United operations home base data on each flight. The SFO United chief pilot was probably waiting for them at the gate when they landed. Airlines will use FOQA data to notify them when overspeed actions or other quality problems occur in flight and will ding the pilots. It's very common.

Also, spacial disorientation is a thing, and the body doesn't typically sense falling very well without visual cues. It is entirely possible that the drop coupled with turbulence on the climb out didn't register with passengers (nor the pilots!) as much as you think. If you've ever experienced unusual attitude recovery training as a pilot, you'll know that it's somewhat shocking what your body is telling vs what you see when you pull your head up and look outside. It is a jarring experience when you learn that your body is straight up lying to you.