C. botulinum can enter the process on raw materials. The spores of C. botulinum are very common. They have been found in the gills and viscera of finfish, crabs, and shellfish. C. botulinum type E is the most common form found in freshwater and marine environments. Types A and B are generally found on land but may also be occasionally found in water. It should be assumed that C. botulinum will be present in any raw fishery product, particularly in the viscera.
Because spores are known to be present in the viscera, any product that will be preserved by salting, drying, pickling, or fermentation should be eviscerated prior to processing (see the “Compliance Policy Guide,” Sec. 540.650). Without evisceration, toxin formation is possible during the process, even with strict control of temperature. Evisceration of fish is the careful and complete removal of all internal organs in the body cavity without puncturing or cutting them, including gonads. If even a portion of the viscera or its contents is left behind, the risk of toxin formation by C. botulinum remains. Uneviscerated small fish, less than 5 inches in length (e.g., anchovies and herring sprats), for which processing eliminates preformed toxin, prevents toxin formation during processing and that reach a water phase salt content of 10% in refrigerated finished products, or a water activity of below 0.85 in shelf-stable finished products, or a pH of 4.6 or less in shelf-stable finished products, are not subject to the evisceration recommendation.
Obviously he isn't eviscerating his fish since the process relies on the fish guts still being in there. But he is using small fish. Since he is starting from frozen, he can probably rely on them not containing preformed toxin.
For shelf stable (non-refrigerated) products they recommend 20% salt (which is what he used, unless the calcualtion for water phase salt is somehow different):
Controlling the amount of salt in the product to 20% water phase salt (wps) or more, to prevent the growth of C. botulinum types A, B, E, and F and other pathogens that may be present in the product (e.g., shelf-stable salted products).
Obviously, 20% salt is a lot of salt, I think you can't use that much for many other kinds of fermentation (e.g. vegetables), because the product would be inedible. But for a sauce, it's fine (soy sauce is ~15%). I wonder what the pH of his final product is. Fish sauce never struck me as particularly acidic.
PS: Not a food scientist, haven't even done a lot of pickling, don't believe anything I write, read the original sources (twice).
https://www.fda.gov/files/food/published/Fish-and-Fishery-Pr...