Aslan's PhD advisor Jurgensmeyer confirmed this with "Though none of his 4 degrees are in history as such, he is a “historian of religion” in the way that that term is used at the Univ of Chicago to cover the field of comparative religion."
According to his website, Aslan received a "Master of Theological Studies from Harvard University (Major focus: History of Religions)", which, while not a current option, was probably much in line . The current "Comparative Studies" option might be close or even identical to the erstwhile "History of Religions" option.
The U of Chicago site describes "history of religions" approach this way (emphases mine):
History of Religions pays self-conscious and explicit attention to problems of epistemology, terminology, category formation, method and motive. Irreverent by temperament and sometimes on principle, it insists that [a] the Western monotheisms should not be the only paradigms and/or objects of legitimate study, [b] religion cannot be reduced to belief, but also includes issues of practices, institutions, communities, habitus and other factors that often operate below the level of consciousness, and [c] interpretation involves critical probing and systematic interrogation of the idealized self-representations of any religious phenomenon.
Those who work within the History of Religions are expected to become thoroughly acquainted with the development of the History of Religions as an academic discipline, and to have a sophisticated understanding of the theories and methods that are relevant to contemporary research in the field. Each student must deal creatively with the tension that results from an emphasis on the importance of historically contextualized studies on the one hand, and of wide-ranging theoretical and comparative research on the other.
Jonathan Askonas wrote on 7/29/13 in "Digging Deeper into Aslan's 'Scholarship'" (emphases mine):
I’ve read [Reza Aslan's PhD] dissertation, and can report that it uses no historical methods or archival research. It solely focuses on the events and movements of the twentieth century, with the exception of one ten-page summary of the life and times of medieval Muslim theologian Ibn Taymiyyah. In the fields of sociology and political science, it seems like a rather unremarkable piece of work (it’s also unusually short for a PhD dissertation, at about 130 double-spaced pages. Dissertations usually run into the hundreds). It also seems likely that much of the research was later published for a popular audience (along with the usual current events punditry) as Aslan’s book Beyond Fundamentalism. Absolutely nothing in the dissertation gives any indication that the author has any interest in, much less qualifications for, New Testament scholarship.
Aside from content, Aslan’s claim that he is a scholar at all is questionable given the publishers of his books. . . . Literally the only remotely academic article he’s published was a 2003 piece in an obscure UCLA law journal on the sociology of stoning in Islam. Again, Aslan has no scholarly work that would qualify him as an expert in New Testament studies by the standard practices of that field.While Aslan has possibly imbibed demythologizing trends from the "History of Religions" school, that does not necessarily mean that his doctorate gives him any convincing expertise in New Testament history or nearly any kind of history.