Do you think it's plausible that SIBO could explain the increase in disease activity after contracting a water/food-borne pathogen in Mexico and the positive response that you had to restricting starch and fermentable fibre and taking herbal antimicrobials, rifaximin, and metronidazole?
100%. Also, you've definitely brought up very important mechanisms that have a very important mechanical action to worsen the environment and cause dysfunction. One thing to note, that isn't noted in that analysis is that any bacteria (good or bad) regulate cytokine expression through interleukin modulation in the gut. Thus, any kind of SIBO is likely going to cause you to house invaders that upregulate IL-17 (the interleukin pathway that deals with insults to epithelial linings - gut, lungs, etc) The IL-17 pathway is massively implicated in AS/SpA's. Thus, we have mechanical issues you mentioned, the immune memory formed that makes infection more likely and inflammation much more potent, and then we have the driving of the inflammatory pathways through the communication the bacteria have with the gut lining itself. All of these things are extremely important in creating the "profile" of an AS sufferer.
To further your line of reasoning, and demonstrate is indeed valid, check out: Link
Basically, I've treated myself under two assumptions:
1) I have massive SIBO due to dysfunction in the immune system, so treat SIBO
2) Keep the number and type of bacteria lower than in normal/healthy people since the number of interactions with the intestinal wall drives the inflammatory pathway. The LPS coating (lipopolysaccharide) of bacteria is one of THE most potent initiators of acute inflammatory (TNF-alpha) response.
So, I may not have as diverse a gut microbiome as I used to, but mainly because my immune system is too hypersensitive to allow it anymore. This might make me more susceptible to various infections (but nowhere near as susceptible as blunting my immune system with biologics), but at least I am more mobile and able to eat a wider variety of food. I think, in the long-run, this is more important as I won't have as many nutritional deficiencies and I can enjoy a more normal/mobile life.