I thought like you time ago.
The small bowel is an almost sterile place. HCL, pancreatic enzymes, bile, sodium bicarbonate released by the pancreas control the microbial growth there. Another thing that doesn't allow is the intestinal transit, the SB is a high traffic area. People experience SIBO have intestinal motility problems.
I had what you may consider a severe leaky gut. I don't want to mention what I have taken to correct it. Image yourself. Go to the archives an began to review my posts to realize what I have done.
The colon is the place where short chain fatty acids are produced. Those are the fuel of the intestinal lining cells.
If there isn't correct production of short chain fatty acids, your small bowel lining atrophies causing leaky gut.
Most of the symptoms you think are caused by leaky gut are antibody reaction to candida toxins. Candida metabolizes carbohydrates and toxins so its metabolic wastes also contain molecules of it that later act as an antigen.
Let quit the theory, I haven't taken a single probiotic, nothing to heal the gut, and I am totally normal. Eliminating the gross of the fungal colonies that were living in my colon, the rest has returned to normal. Who knows the complete explanation ?? What matter is to recover, let god to put the things in the right place.