The API feels wrong. The object that was passed to pub() is the object that should be received by the callback passed to sub().
The use of EventTarget/CustomEvent is an implementation detail; it should not be part of the API.
As a result, every callback implementation is larger because it must explicitly unwrap the CustomEvent object.
Essentially, the author made the library smaller by pushing necessary code to unwrap the CustomEvent object to the callsites. That's the opposite of what good libraries do!
The mentioned nano-pubsub gets this right, and it even gets the types correct (which the posted code doesn't even try).