I am trying to put together a hubsite on ArcGIS online that uses data from both hosted services and those connected in from an on-premises ArcGIS server. When accessing the layers through the data explorer in the hubsite the hosted layers are all available for download but those from the on-premises services are not.
Based upon the following paragraphs in the help pages I would have though this should be allowed.

I have tried adding the services in a a range of ways grouped map services, single layer map service, feature access enabled, but all to no avail.
I am therefore wondering is this at all possible or have I misunderstood the help pages?