In SwaggerHub we are expecting remote $ref to refer to content inside a Domain, not an API flat file.
Create a new Domain and add your remote components to it. Then replace the faulty $ref in your original APIs with the new domain anchor.
type: $ref: and then press control-space, SwaggerHub will show you all the domain anchors that are available and you click the one you want.
The auto-complete is context sensitive. If you are in a place needing parameters, you will only see parameter anchors, if you need a schema control-space will show you all the schemas in your domains. Yes you can have more then 1 domain.