The recommendations on that page don't make much sense? The foods recommended are not major dietary sources of tryptophan (cf. [1]) and while a variety of fermented foods are mentioned, I am not aware of the use of the crucial bacterium Clostridium sporogenes in any deliberate fermentative cultures, such as are used for yogurt or kombucha, which rather contain [checks fridge] Lactobacillus, Bifidis, and Streptococcus species. But Clostridia are typically found in the gut of breast-fed newborns within a week of birth [2], suggesting they don't need much help.

I will also note that there has been some work done on the incorporation of Clostridium sporogenes into probiotics, but it doesn't come from kefir or miso, and... well... just read it! [3]

1: https://en.wikipedia.org/wiki/Tryptophan#Dietary_sources

2: https://jasbsci.biomedcentral.com/articles/10.1186/s40104-01...

3: https://scholarworks.calstate.edu/downloads/x920g252b