While I haven’t read the particular rabbi’s book you’re talking about, I am in fact familiar with the Hebrew perspective you’re talking about, including why they went that route from their scriptural testimonies. The position is so close to binitarian theism that there is practically no distinction, except that they don’t think the El Shaddai aspect of God (which they treat as being practically a second person of the one and only one God) has incarnated as the Messiah (although they do think He has routinely acted as the visible YHWH in many famous and even obscure Biblical incidents. While occasionally interacting in personal distinction with the invisible YHWH.)
I don’t recall if they think God eternally self-generates Himself this way, but many trinitarians wouldn’t say that either (although I do. In technical terms it’s the difference between privative and positive aseity: God simply statically exists, or God actively eternally self-generates Himself.)
When I talk about the subordination of God the Son, and the self-sacrifice of the 2nd Person even at the level of God’s own existence, and how the 2nd Person specially relates to the creation of not-God natural systems and of derivative persons in such systems, and why manifestations of God within such systems would be that 2nd Person of God (not the 1st Person), and why we should expect an Incarnation of God (beyond a mere manifestation of God), and why the Incarnation of the Son would be a pouring out and raising back up (in several various ways) thus becoming perfect with us while in other ways already being perfect, yadda yadda yadda etc. etc. {inhale} 
…
…I’m really only detailing in technical thought what the rabbinic tradition you’re talking about preferred to speak of in poetic thought, and in relation to much the same scriptural testimony.
But they had their own metaphysical disputes on such things, too, precisely because they took seriously the injunction not to religiously worship lesser lords or gods, nor to affirm multiple Most Highs, while also taking seriously scriptural testimony indicating there were multiple persons they were supposed to be religiously worshiping as (and identifying as) YHWH. (Thus, other rabbis contest that El Shaddai, the Angel of the Presence, the visible ADNY, is really YHWH.)
In short, the tradition you’re talking about directly implies that there are (at least) two Persons of one YHWH, in a real (not merely modalistic) personal relationship where one Person is in authority over the other Person, the Person in authority giving all things to the subordinate Person, and the subordinate Person receiving all things from the one in authority, both Persons serving each other in different ways, in what philosophers would call an agent/patient masculine/feminine relationship (even though both Persons would be masculine compared to anything in creation): not two YHWHs, nor a created person pretending to be YHWH as a idol for us to worship. We worship El Shaddai personally, El-elyon na Adonai, but we don’t worship anyone less or other than YHWH Most High in doing so. On the contrary, we are grateful to YHWH, the same El Shaddai from age to age, for giving Himself in sacrifice for our sake.
So you’re not actually referencing anything against trinitarian Christianity there, but actually something far more in favor of it than otherwise. 
(Although the kabbalah material can get pretty extreme–which is why orthodox Jews are careful about using it, if they use it at all. It often goes beyond the boundaries emphasized in the scriptures: boundaries which turn out to have logical rationales, too, for not going beyond.)
Now I think you’re saying that he was making the point that many EUs, who themselves want others to question certain scriptures on God’s salvation of all, now won’t question other scriptures that don’t fit with their current beliefs. Do I have that right? Or will I need even more 
Like I said to Julie, I’m open to either side at this point, since I’m in flux, reevaluating much of what I believe.
I think that we do learn in layers. When I first learned the Bible, I took everything on the surface (and then applied the lenses of the person teaching me). But as you know, the more you study, you begin to realize there are deeper layers that alter the way you perceive the meaning of what you once read on the surface.