"what do you mean, braces don't actually delimit scope?" I would answer, they don't. Because this is not <insert your favourite braces-delimit-scope language>. This is javascript. Nobody should have ever told you that braces delimit scope here.
It's not like this is some extremely rare issue that you aren't going to find documentation for. A quick run through a basic tutorial would suffice.
So how is it that 'let' works the way variables should? It works differently, that's all. So now you have to know how let is different from var, why it was introduced, when to use let, when to use var, etc.
Then when you see code using let, you're going to have to determine whether the person used it because they know what they are doing, or because someone just told them "let is the new var" and they use it all the time. Same for var. There is an added level of uncertainty when reading code. And and added level of complexity in explaining the language. You haven't gotten away from explaining how var works. Because you're still going to see tons of code using it.
Wouldn't it be easier to just learn how to use var properly?