I think what your OP was referring to is not adding a citation to a document that you're editing, but rather ingesting PDFs and bibliographic data from the web.

I love Emacs (not as much as you do it seems ;) but Zotero is amazing for ingesting academic references including fulltext and bibliographic data from the web. One click of a button in your browser, and everything is ready to read and cite.

(Source: Wrote quite a number of papers and co-authored one textbook, all using Zotero. My workflow was Zotero for ingestion -> per-publication bibtex files for authoring)