I am not saying they don't have salt but it's a kind of popular salt replacement. For Asian cooking soy sauce is used in most of Asian cooking and and nowadays

I am not saying they don't have salt but it's a kind of popular salt replacement. For Asian cooking soy sauce is used in most of Asian cooking and and nowadays for example they are three type of popular soy sauces, 1) original 2) sweet and 3) salty, but even the original and the sweet one is very much salty. If you cook with soy sauces you need to adjust for the saltiness and do not put much salt, and some cases with salty soy sauce you don't add salt at all since it's already salty enough. I think this what probably happened, in the old days there's no Monosodium glutamate (MSG) for artificial umami flavour and they probably use garum as a kind of convenient hybrid salt and umami flavour (MSG) replacement. Most of the Asian restaurants now apparently are using MSG for better taste but personally I have found it bad for after taste with my nose feel itchy and dry soon afterwards.