EventBridgeEventObj expects a JSON string in the detail field always as discussed and implemented in #704. I don't know if it's possible for it to actually be a string, but it can definitely be an object. See an example here: https://docs.aws.amazon.com/eventbridge/latest/userguide/eb-events-structure.html.
This makes the types here currently unusable for the most common pattern which is when creating a Lambda that processes EventBridge events from AWS services, like ECS.