He lied. This isn't a huge surprise, or at least it shouldn't be, since Michael Bay also swore Megatron wouldn't be in Revenge of the Fallen (and he was) and because Michael Bay is an enormous douchebag who enjoys lying to audience seemingly for kicks. However, it's worth noting that Michael Bay left a message on his personal website yesterday morning, stating definitively that the Twins would not return in TF3... mere hours before two Chevy Sparks, identical except that one was green and one was orange, in the same shades as the Racist Twins as shown in TF2, were seen being filmed on the TF3 set.
I'm actually kind of impressed. It takes a special amount of douchery to be able to lie to your potential fans so obviously, for no reason, and not give any shit whether you're caught or not. Truly, Michael Bay is a very, very special asshole. Thanks to Enigma2K99 for the tip. (News via TFW2005, art via Seibertron)
PS: Please note that Skids has an Autobot insignia on one of his buckteeth, because Michael Bay hates you. That is all.
UPDATE: So apparently the article I linked to is a year old. Like, it was posted on May 11th, 2010. I clearly saw the first part of the date, and missed the year. My apologies. So Bay could very well be telling the truth, and while he clearly shot footage of the twins in 2010, maybe he's since decided to cut them out of the film. However, he still needlessly lied about Megatron not being in TF2, and I wouldn't put it past him to be lying about this. I guess we'll see.