I was curious about this description too. I often assumed it was to describe the level of consciousness/awareness MLCer had at each point of the "journey"/breakdown/illness.
I saw the metaphor to be: At the beginning and end there is more "light"/clarity, but in the middle it is very dark/no clarity/MLCer's actions are very unconscious and they are unaware of time, chronology, others around them, etc.
I myself acted like someone in a crisis when I had a bad reaction to birth control in my twenties. I felt very disconnected from others, even those I knew I loved and liked. I remember feeling ashamed knowing I was making bad decisions but didn't know how to get out of the bad decisions I had made. While mine involved abruptly switching graduate programs (which involved an international move and a lot of wasted tuition money), I remember telling a friend something along the lines of, "I have found my niche!" just like my MLC-H suddenly found his "niche" with OW. It was a kind of fanfasy/wishful thinking that turned out not to be true, because it didn't solve my internal discomfort and I soon felt just as disconnected to this new field of study as I had to the last. It was like I couldn't "commit" to anything.
It really was a sense of "dis-ease" that RCR describes. It wasn't mental only, it was a constant physical tension. I guess if I look back I don't know if I would describe it as a tunnel -- maybe a different vibrational frequency in some ways. It went away as soon as I stopped taking the pills, although I guess there were some identity issues as well. It truly very unconscious, like a fog. I moved several times, and two of those times I remember having moments of not understanding why I had chosen the living situations I found myself in.
I don't know if that will help, but it may explain how a person can act out of character because of a biophysiological reason plus identity issues, feeling aware of their poor decision-making but unable to extricate themselves.
It is true that there is likely nothing anyone could have said or done for me at the time to help me out of it, but the kindness and acceptance of my family helped me. Judgments and criticisms really did just push me farther in as I was "hiding" in many ways.
When I stopped taking the pills I went to visit my parents in the country I grew up in. I felt this trip reconciled a lot of the identity issues and gave me more of a foundation. But again -- I only made this good decision for myself once I was not under the influence of synthetic hormone (ie likely depressed).
Modification: I am now really thinking of this period and remembering (still feeling ashamed) that I monstered and blamed my parents many times almost in exact same way my MLC-H monsters/blamed me. It did not help when they argued back or got angry with me because I felt truly wretched and misunderstood. I still feel gratitude that when I finally went to visit my parents they treated me with so much love and acceptance. It really was like they gave me/I found the "home" that was missing in my heart.
Another thing, just like a MLCer, I avoided many people from my past. I felt ashamed, a failure, like an outlier, for things I now realize all people in their twenties go through. It's actually helpful for me to remember this as I can use this when interacting with my MLC-H.