Oh man the the amount of fish sauce he wants you put in this will ruin it. Add a teaspoon or a tablespoon or something, but not 1/4 cup. The first time I made it I nearly threw it out.
It's a tablespoon per serving which isn't that crazy at all. Also Vietnamese fish stock is milder than Thai fish sauce if that's what you're thinking of.
Go through the SE comments on this recipe. The amount of fish sauce makes the broth smell like asshole. I recall several Vietnamese scratching their heads over it. I love fish sauce, but Kenji went nose blind to it or something.