Hm... I'm not sure why it would imply that "identical to" is incorrect -- it is idiomatically correct, as is "identical with". See this quote from a commonly cited grammar reference: "identical takes either with or to. Historically, with has been considered better because one has identity with something or someone, not to it. Identical to was not widely used until the mid-20th century. The OED's illustrative examples contain only the phrase identical with. But today, especially in American English, to predominates." -- From the Oxford Dictionary of American Usage and Style. Basically "with" is an older use, but "to" is also now used and accepted.
Do they directly say that "identical to" is incorrect, or is it just in an incorrect answer choice? Generally you are going to always be able to find an alternate reason to eliminate an answer choice other than an idiom. Focus on pretty much any other decision point first, and chances are you'll either eliminate the incorrect idiom or you'll find that the idiom was a 'false decision point' (there are sometimes two correct idiomatic usages of the same word) all along.