It should be noted that the Dictionarium, which was written contemporaniously, does use y for the semivowel.
It has taken the allied sound of the semivowel y, and so, with the preceding vowel, constitutes a diphthong.
In roi it is the semivowel allied to series p; in oil it is the semivowel allied to series k.
Effect of the semivowel y on certain letters when they precede it.
The diphthong oi is the vowel o modified, plus the semivowel y (not the vowel i) modified.