It would be lying if Takaki knew that beriberi wasn't caused by germs. But back in the 1870s, he didn't know that. He only knew that something in barley was he

It would be lying if Takaki knew that beriberi wasn't caused by germs. But back in the 1870s, he didn't know that. He only knew that something in barley was helping to prevent beriberi, and he was in fact mistaken about what that something was. (He thought it was proteins when it was actually vitamin B1.) So the Antibiotic Barley Theory could have been a reasonable hypothesis. Vitamin C is named "ascorbic" (i.e. anti-scurvy) acid because the people who discovered it were looking for a substance that prevented scurvy, no matter what the mechanism of action. The mechanism of action was only known for sure after the substance was identified.