They said they did an "investigation". They did not say whether they investigated the actual hardware or just their complaints or repair statistics.
Its common sense that any "Apple led investigation" that would result in huge costs in repairs would not be based on complaints alone. I did read that they tested enough GPUs to determine that the concern was valid. Its a certainty that complaints and repair statistics sparked the investigation. Extending a warranty for an entire year for ALL notebooks built in that time period is not something they would take lightly or on word of mouth.