I would normally agree with you, but John Kerry was not just any soldier. John Kerry came home and led a major movement against US policy in Vietnam. For that reason, a person like Lehman might very well remember Kerry and any citations he may or may not have given him.
BTW, at least in this article, there's nothing said about Lehman simply not remembering the citation. In fact, his denial of the citation seems pretty emphatic, to the point that I believe he remembers Kerry quite well.