Are contributions to the American Political Science Review labeled "Letter" peer reviewed? I suspect that in general, they are not, but perhaps someone who knows better could clarify.

However, I am looking specifically at the American Political Science Review, where contributions labeled "Letter" are much longer than is typical in other journals. Are these "Letters" peer reviewed?
Yes, they are peer-reviewed.
See also their author instructions:

The American Political Science Review (APSR) publishes letters, or research notes. [...] Although letters are reviewed like manuscripts [my own emphasis], they are shorter, more focused, demonstrate a novel perspective on existing research, and encourage scholarly debate [...].