I would guess that this is a Revit restriction- even though you can see a lot of the information through the API, you are limited in the UI to what you can copy from the linked file.
However, through the UI, you are able to cross-doc copy/paste if you have both files open.
I would suggest trying to do the following through Dynamo:
- Temporarily unload the link (as Revit will restrict you opening a linked file in the same session)
- Open the linked file
- Select the desired sheets/views to copy
- Paste them into the “host” file.
- Close the linked file
- Reload the link
Also, there is a fairly lengthy discussion on the subject here (they start off talking about floor plans, but there is discussion about copying detail sheets further down):