Academic writings tend to offer a broad spectrum of views on any issue, particularly when it comes to journals which are concerned with the quality of justification for a viewpoint rather than how correct it may be in practice. In either case, an academic critique of the law is not the law as it stands today and it is incorrect to state that duress is at this time (although it may be reclassified so in the future) a justification, when plainly in jugements such as most recently in R v Hasan and in R v Wilson that duress is refered too expressly as an excuse.
In case it should interest you, obiter statements in the recent case of R v Wilson (2007) appear to be expressing the appeal court's displeasure with the hard line taken in Hasan and may well be the basis for some future appeal which might provide yet more clarity.
I remain resolute that the wording 'excuse' as it is used in ratio decidendi, should be correct term applied in the article, rather than 'justification' which while there may be an arguement for is not the terminology being applied in practiced law today.