There is IEI atheists you know, so describing Ni as 'mystical' or even 'spiritual' makes little sense.
Sure that's how it appears, but all Ni is - is introverted intuition. Which lends itself to deep reflection, self-analyziations etc, IEIs being excellent writers and philosophers, and able to understand/comprehend themselves and other individuals. Ni ego also accurately connects many ideas together, leading itself to what we call as creativity. It is timeless in a sense, compared to the very timely mechanics of Te. Just think, you can gather great insights from IEI stories even to do this day, and they can empower you (or have some other effect). But because efficiency always has to be changed/upgraded (more than the IEI can keep up with), a new manual/guidebook has to be updated quickly.
And since Ni makes us feel powerful, living in these timeless worlds isn't as painful as it would be to other types, so Eckhart's book, and a lot of other things- basically any guidebook telling IEIs to 'stop being so emo or fantasical, or live more in the present' isn't going to work too well. Eventually, the IEIs own Ni will get bored and they will seek 'to do something' by ourselves, but still when an action is accomplished, when IEIs are simply living, it will always be reflected later to try to make some meaning off of it. Otherwise how could we accurately learn from history?
If we want to take socionics seriously, we kinda have to cut out the mumbo jumbo talk. Yes, there is such thing as estoerics, 'walking in between worlds' but it's a completely mental activity, there's nothing truly supernatural about it, even though it looks that way.