There are two problems. First, BeDuhn adopts a 4th century, and thus anachronistic, notion of canonicity to characterize Marcion's anthology of scriptures, i.e., the evangelion and the apostolikon. He is unwilling to consider that what Marcion produced was, in reality, nothing more or less than a book (anthology), and insists on elevating the historic and theological importance of this publication by calling it a canon:
This book is not about Marcion, but about the canon of Christian scriptures he introduced as the new touchstone of Christian faith. Before Marcion there was no New Testament, with him it took its first shape, and after him it gradually developed into the form we now know.
...Before Marcion there were Christian writings that were read and treated as, in some sense, authoritative. But they had limited, local circulation [how can BeDuhn claim to know this?] and were not incorporated into a larger Bible.
...So it was that Marcion collected, for the first time in history, a set of authoritative Christian writings intended to be afforded a status above that of other Christian literature. [6]
Marcion formed for the first time "a coherent canon," displaying two crucial features: (1) it contained a fixed number of books, and (2) it was put forward in place of the Jewish scriptures, as equivalently scriptural. Through these moves, he "first makes Christians conscious both of the idea of a new canon of Christian literature and of the identification of certain kinds of documents as carrying greater authority than others, and hence being 'canonical'" (quoting Metzger, The Canon, p. 98). [26]
There is much to agree with here, but I think the "canon" language is overblown. For one thing, the actual words "canon" or "canonize" were not applied to the Christian scriptures until the time of Athanasius. And in that context, as I have already argued, it was the political and legal environment in which some scriptures and not others were allowed to be read in the imperially sponsored churches, that gave meaning to the idea of canonicity. For example, the Council of Laodicea in 363 CE declared that "only the canonical books (τὰ κανονικά), as opposed to the uncanonical books (τὰ ακανόνιστα) ought to be read in Church" (Metzger, Canon of the NT, p. 292). (See Metzger's Appendix on the history of the word 'canon'.)
In the 2nd century, a time of prosperity and much bookselling in the empire, many Christian books and collections of books were written and published. How do we know that Marcion's was all that special? Certainly, it was very successful, i.e., read and believed in. Every book that is ever published hopes to take the world by storm, but it hardly makes sense to say that, by publishing an anthology, a person is thereby fixing a canon. The fact that there were a "fixed number of books" is hard to avoid for any anthology. However we wish to characterize Marcion's intentions in producing the book, it is much better to attribute its historical impact and importance to its reception. The reception of a text is what makes it normative or canonical, not the publisher's intentions.
The second problem is that Beduhn seems to take the empirically problematic stance that our modern NT did not come into existence even as a book until the 4th century. In other words, he conflates the formation of the NT with its canonization, ignoring the evidence provided by Trobisch that a NT text, titled "the New Testament," was in circulation throughout the Mediterranean by the 3rd century, and most likely was first published in the 2nd century.
BeDuhn holds that the title "New Testament," which he wants to ascribe to Marcion, is otherwise little in evidence until the 4th century. But this seems to be wrong. We have already noted the anonymous anti-Montanist writer who explicitly refers to a written text as "the New Testament" in the late 2nd century. Trobisch also cites references in Clement of Alexandria, Tertullian, and Origen (First Edition, p. 44).
It isn't necessary for there to be universal agreement on a particular collection of orthodox scriptures in Tertullian's day in order for him to accept them as "the true scriptures," and to presume that they are the older and authentic scriptures by comparison with Marcion's. Again, I see here a conflation by BeDuhn of what is a canon ("universal agreement") with what is merely a published anthology.