(Did you know that dates don't cause tooth decay? Yes, the study concerned Middle Eastern school boys, iirc.)
I've never heard this before, though I have heard the exact opposite. While I can't remember where I read it it, it was something where someone was discussing middle eastern diets and they mentioned that some children, unfortunately, would overeat on dates like they were candy at the expense of other foodstuffs and that they had the more conventionally expected results, ill health, bad teeth.