Thank you for asking your question on Just Answer,
In answer to your question yes an external Excel spreadsheet that is linked into a word document is a considered a compound document.
a compound document is a document type typically produced using word processing software, and is a regular text document intermingled with non-text elements such as spreadsheets, pictures, digital videos digital audio, and other multimedia features. It can also be used to collect several documents into one.
Yes a web page is also considered a compound docuement due to it holds text and individual objects like pictures, sounds, videos, Java applets, and ActiveX controls.
Here is a link to some additional documentation regarding compound documents,
Hope this helps,