This is actually simple.
No, AT&T did not specifically intervene in the google app approval process. They didn't have to... It was in Apple's contract with AT&T to not allow IP calls, a term negotiated during the strong-arming apple used to get unlimited data for $20 including 200 free texts in the data plan (later changed to $30 without texts). Keep in mind, the original contracts between Apple and (Cingular) were done when the iPhone had no 3rd party apps at all. AT&T had already negotiated their changes to the cingular contract before the 3G and the SDK were released. (they had been negotiating deal changes since the day the bought Cingular).
Apple, under AT&Ts non disclosure of their mutual contract terms, could not publicly communicate the reason they had to previously reject google's app, so they made up the "replaces functionality" thing.
AT&T really is the reason google could not be approved, ecven though they were not directly involved. Likely, googles use of some internal functions were also questinoable, especially including the address book, and may have contributed. The fact it could directly dial calls from within its own interface, including auto-answering the incoming call connection, might have been an issue too. Finally the ability to potentially hide incoming callers behind false callerID put a kink in AT&T's "A-List" plan, something appel would I'm sure also be held to protect under secret contracts.