Plz see embolden phrase under. Why's argument 1 "sleight of hand"? I don't grasp why author brings up international law – how's it relevant? This case is about how the UK "had not ‘implemented’ the directive into national law.".
Robert Schütze. European Union Law 2 ed. 2018. p 96.
a. Direct Effect and Directives: Conditions and Limits
That directives could directly give rise to rights that individuals could claim in national courts was accepted in Van Duyn v. Home Office.93
The case concerned a Dutch secretary, whose entry into the United Kingdom had been denied on the grounds that she was a member of the Church of Scientology. Britain had tried to justify this limitation on the free movement of workers by reference to an express derogation within the Treaties that allowed such restrictions on grounds of public policy and public security.94 However, in an effort to harmonise national derogations from free movement, the Union had adopted a directive according to which ‘[m]easures taken on grounds of public policy or of public security shall be based exclusively on the personal conduct of the individual concerned’.95 This outlawed national measures that limited free movement for generic reasons, such as membership of a disliked organisation. Unfortunately, the United Kingdom had not ‘implemented’ the directive into national law.
Could Van Duyn nonetheless directly invoke the directive against the British authorities? The Court of Justice found that this was indeed possible by emphasising the distinction between direct applicability and direct effect:
[B]y virtue of the provisions of Article  regulations are directly applicable and, consequently, may by their very nature have direct effects, it does not follow from this that other categories of acts mentioned in that Article can never have similar effects. It would be incompatible with the binding effect attributed to a directive by Article  to exclude, in principle, the possibility that the obligation which it imposes may be invoked by those concerned. In particular, where the [Union] authorities have, by directive, imposed on Member States the obligation to pursue a particular course of conduct, the useful effect of such an act would be weakened if the individuals were prevented from relying on it before their national courts and if the latter were prevented from taking it into consideration as an element of [European] law. Article , which empowers national courts to refer to the Court questions concerning the validity and interpretation of all acts of the [Union] institutions, without distinction, implies furthermore that these acts may be invoked by individuals in the national courts.96
The Court here – rightly – emphasised the distinction between direct applicability and direct effect, yet – wrongly – defined the relationship between these two concepts in order to justify its conclusion. To brush aside the textual
argument that regulations are directly applicable while directives are not, it wrongly alluded to the idea that direct effect without direct application was possible.97 And the direct effect of directives was then justified by three distinct arguments. First, to exclude direct effect would be incompatible with the ‘binding effect’ of directives. Second, their ‘useful effect’ would be weakened if individuals could not invoke them in national courts. Third, since the preliminary reference procedure did not exclude directives, the latter must be capable of being invoked in national courts.
What was the constitutional value of these arguments? Argument one is a sleight of hand: the fact that a directive is not binding in national law is not ‘incompatible’ with its binding effect under international law. The second argument is strong, but not of a legal nature: to enhance the useful effect of a rule by making it more binding is a political argument. Finally, the third argument only begs the question: while it is true that the preliminary reference procedure generically refers to all ‘acts of the institutions’, it could be argued that only those acts that are directly effective can be referred. The decision in Van Duyn was right, but sadly without reason.