I don't want to enter a battle of reference, but let's agree it's not so "clear":
> The oldest evidence of the production of soy milk is a Chinese mural carved into a stone tablet. It shows a kitchen scene, proving that soya milk and tofu were produced in China in the period 25-220 AD.
Soy milk is a byproduct of tofu, which is not fundamentally more difficult to produce than cheese and others animal milks products. It can even be made without tofu: grind some seed and let them soak in water. I'm not an expert on ancient technologies but it doesn't seem more complicated than taming a squeezing the nipples of a wild animal.
> The oldest evidence of the production of soy milk is a Chinese mural carved into a stone tablet. It shows a kitchen scene, proving that soya milk and tofu were produced in China in the period 25-220 AD.
https://www.soya.be/history-of-soy-milk.php
> it first appeared in A.D. 82
https://www.soyinfocenter.com/HSS/soymilk1.php
Soy milk is a byproduct of tofu, which is not fundamentally more difficult to produce than cheese and others animal milks products. It can even be made without tofu: grind some seed and let them soak in water. I'm not an expert on ancient technologies but it doesn't seem more complicated than taming a squeezing the nipples of a wild animal.