The NUMMI plant produces the Toyota Corolla, Toyota Tacoma and Pontiac Vibe, and while it's considered a joint-venture between Toyota and GM, the plant is run using Toyota's manufacturing process. From what we can tell, it operates much like a training ground where GM managers can go to learn Japanese manufacturing techniques. Thus, these allegations appear to be aimed more at Toyota than GM. The response from Toyota has been that it's "tackling quality problems as a top priority," while GM wasn't aware of the lawsuit at the time the linked article was written.
Toyota's sterling reputation for quality has been tarnished lately with recalls becoming more common and Consumer Reports removing a few of the Japanese automaker's vehicles from its recommended list. We imagine their must be tremendous pressure at the plant level to fix any quality issues that arise, which could encourage managers to make some questionable decisions. While we don't know for sure what happened at the NUMMI plant in California, the idea that some managers would choose to hide defects rather than report them doesn't seem that far fetched considering the pressure to be perfect.
[Source: AP via Wheels.ca via ImportCarCanada, photo via Inside Line]