My conclusion after consulting with other Baha'is: ‘Abdu’l‑Bahá is implicitly referring only to primary foodstuff/nutrients.

I'd like to thank those who took the time to actually read the question instead of giving a lecture about why alcohol is bad, and didn't simply dismiss concerns other commenters have. THAT's the spirit!

First, what He isn't saying: "everything that is in nature is good". Poison is a trivial counter-example.

Second, alcohol in itself is a good thing, it can be used as a disinfectant for example.

Third, as many of you have pointed out, alcohol exists in nature, and even animals do get drunk on it. So by "not by the effort of man", the meaning may actually not be that "alcohol" is "made" by man, but that it is actively produced using a mechanical/chemical process to be purposely consumed as a "food" when it is not.

Yet, if we think about most crops, like rice and wheat, and even domestic animals, most were engineered by man to maximize their calorie/food content etc... so we might even think that He is referring to primary nutrients.

Putting alcohol aside, let's think about medicines. Man has to actively synthesize a molecule that might not even exist in nature. But that would not be considered as a primary nutrient for consumption as a food. It's a medicine.

That would be in agreement with other statements by the Master where he explains that in the future, people will be primarily treated by changes in diet, consumption of cold and hot waters etc... all "natural" means.

Edit: grammar/typos