I think you're reading this completely wrong.

The FBI asked for access, Apple said no (because they knew the case was about precedent rather than capabilities). Apple knows the older phones had vulnerabilities (see this faux-apple computer - https://youtu.be/zsjZ2r9Ygzw?t=15m50s commercial).

This is the follow-up from Apple saying "oh, you needed our help to crack it huh? How did you suddenly find a way to do it on your own without us as soon as you realized public perception wasn't proceeding as you hoped?

EFF seems to think that the FBI is legally required to disclose the method (https://www.eff.org/deeplinks/2016/03/fbi-breaks-iphone-and-...) due to their VEP process.