Why would a footnote be considered less credit than something in the list of references? It's pretty common in my field to use footnotes to give credit to other researchers for ideas that arose in an informal context, e.g., "I thank A. Zee for pointing this out". Indeed, the question of whether prose footnotes are at the bottom of the page rather than displayed intermixed with citations to published sources (as in Physical Review Letters) is just a stylistic choices that varies by journal.
Obviously unpublished sources can't easily be counted into automated citation tracking systems for technical reasons, but if the author wanted that they could publish something (or, e.g., put it in a free repository like the arXiv).