Here's the situation. The max extension that Bynum could have gotten from the 76ers in August would have been 2 years due to CBA rules which wouldn't have allowed the 76ers to sign him to a 3 year extension until 6 months after the trade. If he and 76ers did agree to extend, it would have been planned to take place in February if it was to happen at all.
Here's the ProBasketballTalk article mentioning the February 10th details.
As of Feb. 10 (six months after they traded for him) Bynum could sign a max extension, which is three years at just shy of $60 million.
Here's the CBA FAQ mentioning the 6 months clause.
Conversely, a team cannot extend a player it receives in trade for six months, if the extension exceeds the limits of an extend-and-trade1.
In other words, Bynum passed up on a 2 year extension if the extension was even offered by Philly in August or September. I doubt that Philly was really interested in only keeping Bynum for 2 years anyway, especially since they could offer him 3 years in February, so my guess is that an extension was never offered. A 2 year extension wouldn't have made sense to Bynum or Philly at that time.
Bynum's injury issues this season have probably brought any talk about an extension between Bynum and the Philly FO to a halt, if there were any in the first place.