All you say is valid. I copied uncritically from a _The Glories of St. Joseph_. I'm sure that you will find some contextual explanation therein that will make sense of this provoking statement.
-Theo
Padre Giovanni is less known; in fact I could only find a reference to him that is not taken from Thompson, in Italian. But it is interesting:
The Franciscan John of Cartagena (+1618) highlights that "this trinity of people has brought about our salvation: Jesus, as the author of salvation, Mary, as a mediator, Joseph, as a cooperator." (Google Translate)Il francescano Giovanni di Cartagena (+1618) mette in evidenza come “questa Trinità di persone ha operato la nostra redenzione: Gesù, come autore della salvezza; Maria, come mediatrice; Giuseppe, come cooperatore”.
So to him, there is (I assume) the Holy Trinity and then a "trinity of persons", that only shares one person with the Holy Trinity, Jesus, but is otherwise a group of three people.
I think that Thompson took the term "order of hypostatic union", originally meant to indicate a varying degree of proximity to Jesus, to mean an absolute, not relative, hypostatic union of Jesus Mary and Joseph. To say that Mary and Joseph enjoy a special closeness to Jesus is to speak the truth; to say that this puts the three in a hypostatic union qualitatively different from the beatific vision all the saints share, -- is heresy. Does Thompson actually say the latter is not clear, but at least it would had been incumbent on him to clarify this point.