I use a very thin pillow when sleeping on my back. I find sleeping without pillow a bit uncomfortable. Although I am sure I could get used to my head being pushed back a bit, I still dislike the head just resting on the hard mattress and trying to naturally roll left/right. The pillow helps me keep my head looking straight ahead (almost vertically).
As to mattress, I prefer a mid-to-hard latex mattress with a cover.
In the article, the lookout position involves the lack of a pillow. One can imagine that the additional height of the pillow would cause you to raise your head, lower your chest, and arch your back, potentially causing the issues you reported.
That being said, this is an article of anecdotal observations by a physiotherapist - it is not the result of any kind of study.
It is really thin pure goose down pillow. Folded, it is less than 1/4in when compressed. I don't think the height of the pillow makes a huge difference.
As to mattress, I prefer a mid-to-hard latex mattress with a cover.