As much as I can gripe about stupid things that Boeing has done. There is no proof that Boeing did anything wrong here. And it may be long time before we know what happened and why.
The 787 is a very good airplane with an excellent safety record.
The plane in question was far from new and has been in service for quite some time so not very likely the incident was due to aircraft defect.
If the incident was as high in negative g as it sounds, it’s a treatment to the structural integrity that there were no structural failures.