Ellie, I have no idea what SIBO is - whether it is a known and measurable physiological illness, or yet another syndrome that docs have put a self-descriptive label on because they don't have either a cause or a cure.
What I believe is that a body that is exposed for too long to stress, trauma and/or emotional repression, is not a healthy body, and will be more susceptible to actual physiological damage - either by outside invaders (bacteria, viruses), or by cancer, or by the immune system turning on itself.
Dr. Gabor Mate, MD in his excellent book When The Body Say No explains how this can happen in somewhat technical terms, but also with great compassion and wisdom and beautiful writing. I highly recommend it. The subtitle is "The Stress-Disease Connection". No mention of Dr. Sarno or what we call TMS, but it's all about the long-term consequences of emotional repression - the relevance is obvious.
I ended up with RA a year ago - late in life, sudden-onset, with zero family history of any autoimmune conditions. I blame several years of world affairs, then the pandemic, then a volunteer activity that totally went out of control due to the pandemic. Once I had a diagnosis, Dr. Schecter himself told me that I had to do whatever my rheumatologist told me to do - but that I could also certainly attribute the onset to stress, and work on that angle at the same time that I behave myself and take the meds, because he has seen remissions from RA.