I also initially thought that the women were just using a different vernacular than we're used to, and this was their way of expressing how a natural birth was.
BUT I got curious about The Farm and ended up reading some articles about it, and found the
Farmie blog, which is a really interesting first-person memoir of life in the Caravan and on The Farm. While it's only one man's perspective, it really helped to give me a little more context for Spiritual Midwifery. There was a lot of use of various herbal substances and I'm not entirely convinced that laboring women weren't using them. Not saying they were, just not convinced that they weren't.
I personally don't find Spiritual Midwifery to be a terribly useful book for my own birth preparation, but I do think it's a fascinating historical document.

As bryonyvaughn pointed out, how women respond to SM seems based on our own beliefs and experiences - it's possible that I would have found it a more inspiring book from a birth-planning perspective if I had read it a few years ago.