ES claims the following (twice):

Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function.

We don't normatively identify the interface object as a constructor, even if it has a [Constructor]. I guess we should.

A less obvious question is what we should do if there's no [Constructor]. Right now, I think we're aiming to install a [[Construct]] internal method that always throws, but that makes the interface object pass IsConstructor().