So no link stating that the ATS is to be compared only to the significantly more expensive 3 series and C-class. Just as I thought.
Cadillac saying that the 3 and C-class are among the competitors does not mean that it's to the exclusion of all others. The 3 and C-class have bloated in size with the latest generations. BMW and Mercedes added smaller/cheaper models to compensate. The ATS ends up straddling the 2/3 and CLA/C-class.
The ATS is the cheapest Cadillac available. The 2 and CLA are the cheapest BMW and Benz sedan/coupes available, yet the contention here is that they aren't competitors? A base ATS rwd is definitely going to be cross-shopped with the 2 and with the CLA as well as the 320 and C300. Being smaller, especially for fat Americans, is going to put it at a disadvantage to the larger cars, but ahead of the smaller ones.