Danny didn't trade for Davis so he must not have done his due diligence is a false equivalent. Ainge did his due diligence and decided making the move was not in the best interest of the Celtics.
Ainge gets involved in just about every big trade that could be happening. Heck, there were reports he was talking to Houston, when the Davis deal was still an ongoing thing. Obviously, he was looking at other options, i.e. doing his due diligence.