It's very easy to get to slightly below 0g by just pushing on the stick or yoke, and that would be enough to cause what the passengers describe here. You could even get well below 0g. Almost all turbulence of the sort that you can experience in flight are not even 10% of the durability limits of the airframe.
EDIT: I was also thinking about "well why should the airplane allow the pilot for such movements then?". And I think a good analogy is the brakes in your car - they do allow for maximum breaking, yet when was the last time you actually pushed it to the max?
(And note that this is 100% speculation, I just wanted to highlight that the pilot can cause such negative acceleration on their own)
Both 777 and 787 are full fly by wire, with the latter being unstable without computer help due to trimming of control surface sizes for greater fuel efficiency.
EDIT: I was also thinking about "well why should the airplane allow the pilot for such movements then?". And I think a good analogy is the brakes in your car - they do allow for maximum breaking, yet when was the last time you actually pushed it to the max?
(And note that this is 100% speculation, I just wanted to highlight that the pilot can cause such negative acceleration on their own)