So was the secret ingredient... the tiny amount of finely minced fresh onion?
Looking at the final recipe they came up with, that's the only one that seems like you might not guess.
Garlic powder is the only other non-standard ingredient, but I feel like whenever you're looking for the "missing secret ingredient" in a savory dish, garlic powder, onion powder and mustard powder are the first things you'd think of, along with MSG and very occasionally nutmeg or pickles/pickle juice. (While paprika can only be secret in darkly colored dishes, otherwise you can see its specks.)
freshly minced onion does wonders in more than a few recipes, it certainly can add that final kick to tartar sauce if not any recipe with a good creamy base. you don't even need to add much.
I think the point of that quote from the guy is that he realized what it was but he's not ever going to tell. The recipe in the article is the guessed one not the secret one.
Looking at the final recipe they came up with, that's the only one that seems like you might not guess.
Garlic powder is the only other non-standard ingredient, but I feel like whenever you're looking for the "missing secret ingredient" in a savory dish, garlic powder, onion powder and mustard powder are the first things you'd think of, along with MSG and very occasionally nutmeg or pickles/pickle juice. (While paprika can only be secret in darkly colored dishes, otherwise you can see its specks.)