As you like to say in your book, Dennis (and I also like to say in my classes), the Facebook status of this question is, “it’s complicated.”
Certainly Dr Sung presents Plantinga as a concordist (on the basis of a few quotations from his 2011 book, Where the Conflict Really Lies, and she obviously thinks of herself as a concordist, too. That much is not complicated.
To say more with similar confidence, however, I’d need to know a lot more about Plantinga’s views (not just that one book) and hers. I’ll say a few things less confidently below.
First, what does Dr. Sung mean by concordism? My guess is that she means something like this: http://biologos.org/blogs/ted-davis-reading-the-book-of-nature/science-and-the-bible-concordism-part-1, in other words one or more varieties of the OEC view. That’s basically what concordism has come to mean, for the most part, when discussing origins. She does appear to endorse an OEC view, she likes the term “concordism,” and I’m pretty sure I understand her on this much.
Second, is Al Plantinga a concordist in the same sense? Perhaps he is, but if so it’s not obvious from the book she quotes as a whole, much less from the specific passages she quoted. Those passages are about a “deep concord” (which is the title of one of the chapters) between Christian theism on the one hand and the enterprise of science on the other hand. I fully agree with the basic claim here, BTW; one of my current traveling talks is called “Why Christianity Is Good for Science,” and I make historical, philosophical, and theological arguments to support the same type of “concord” that Plantinga also sees.
But that’s not “concordism” as I spoke of it above. One might use the word in that way, but it’s not a common usage.
However, I suspect from other writings that Plantinga is also a concordist of the same type as Dr. Sung appears to be. I’m pretty sure he believes in an OEC view–he’s certainly not a YEC–and I’ve heard him (verbally, in semi-private contexts but still in front of a couple dozen people so I won’t hesitate to talk about it) raise questions and make statements about sin, the flood, and the fall that are entirely consistent with a quite conservative OEC scenario. If someone out there wants to cite chapter and verse from his writings, either to support this claim or to contest it, I’m all ears.
Furthermore, his deep sympathy with ID and his public call for ID proponents to embrace what he calls “theistic science” really do make him sound like a concordist in the usual sense. See http://www.asa3.org/ASA/PSCF/1993/PSCF3-93Hasker.html
That’s the best I can do.