Because like most everything that comes from that guy, it's not accurate.
The fact is that the 2007 offense was "fantastic" the year before. Parcells set that table for Garrett, putting the pieces in place. The offense was already ranked 4th in points scored before Garrett ever got here. He 'managed' to take it up to 2nd his first year here, and then tanked it to 18th and 14th for the next two years. Does that look "fantastic" to you?
Bill Callahan brought them back to top 5 respectability and was pushed out by Garrett, likely because he was threatened by the presence of a better coach.
You can check out the real numbers for yourself here:
https://www.pro-football-reference.com/teams/dal/