Proposed resolution
The widget form element only contains the field name which is not enough to retrieve the field configuration at later hooks. For example in the chosen module we would need the field configuration to be available in the pre render function. To be able to call the \Drupal\field\Entity\FieldConfig::loadByName() function we need the above data.
Proposed resolution
The entity type and bundle values can be added in the form function of the WidgetBase Class.