I was expecting some evidence, such as a scientific study. Instead, all we have is someone merely claiming that it's a myth, which is no better than someone inventing a myth in the first place.

Bright light contracts your pupils. The smaller diameter leads to a greater effective depth of focus. For people who have difficulty focusing on close items, this can help, I believe.

It doesn't mean dim light is "destroying" your eyesight. However, it could be reducing the effective depth of focus, making it difficult to see what you're looking at clearly.