Why did Pope Pius XII mention the soul of Mary? In Munificentissimus Deus Pope Pius XII defined the dogma of the Immaculate Conception:


  By the authority of our Lord Jesus Christ, of the Blessed Apostles Peter and Paul, and by our own authority, we pronounce, declare, and define it to be a divinely revealed dogma: that the Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory.


I can understand the need to define the fact that the body of the Blessed Mother is in Heaven, but was Mary not already a canonised saint before the document was produced? 

Even the protestant reformers did not contest the sainthood of Mary.
Canadian Thomist philosopher Charles de Koninck was a big defender of the Assumption. He saw Munificentissimus Deus (1 November 1950), which Pius XII promulgated only a few months after his anti-Modernist and pro-Thomist encyclical Humani Generis (12 August 1950), as contra Cartesian dualism (mind-body dualism),* since the Pope stresses the unity of the body and soul in proclaiming our Blessed Mother was assumed both body and soul into heaven.
*Cartesian dualism says that there are two types of beings, utterly disconnected from each other: &quot;thinking things&quot; (res cogitans) and &quot;extended things&quot; (res extensa); cf. Four Ages of Understanding p. 523 or the more popular account, ch. 2 of Philosophy of Mind by Dr. Ed Feser.
From De Koninck's &quot;The Immaculate Conception and the Divine Motherhood, Assumption and Coredemption&quot;:

THE SIMULTANEITY OF MARY’S DEATH AND ASSUMPTION
The person of Mary, being merely human, depends, for its reality, upon the actual union of body and soul.  If, then, her soul had been maintained in a state of separation; if, during some course of time, she had been in death, she would have ceased, for that time, to exist as a human being, as a person, and as Mother of God.  Such a state would mean that, during some lapse of time, however brief, the grace of Motherhood—the seat of her habitual grace and of all her privileges as Mother of God—would have been withdrawn.  Now, “what is bestowed through God’s grace is never withdrawn except through fault.”  But Mary, conceived Immaculate and wholly confirmed in the good, could never have committed the slightest of faults. For this reason, we have argued elsewhere that no lapse of time occurred between Mary’s death and Assumption, in which her soul would have been in a state of separation from her body; but that, on the contrary, her resurrection from death took place in the same instant as her death itself.