More to the point, what is the truth? Google leadership post 9/11 knew what "certifying" their communication systems meant and took the money to do it. Whether they knew that the program was named Prism is irrelevant, in my opinion.
Google responds to legal requests for information. The whole PRISM scandal was indicating that the NSA had some kind of direct link into the databases themselves. This assertion is what Google denied, and still denies.
There's been a bit of a wandering definition for PRISM, from "they have NSA software with root access to all machines!" to "they receive LEO requests for information, which they review, and sometimes fulfill." The former is untrue, and the latter is true.
Go look at the slides. The NSA isn't lying in its own internal documents. Of course google is in on it. Do you get how huge this infrastructure is ? Its a massive engineering effort to manage that kind of information flow.
What has happened here is google has got scared. Because without trusting users all their business models fall apart. So they are lying. Its that simple.