The article is from the recently published American Journal of Numismatics (vol. 26). The author makes some interesting points:

The coverage of numismatics in the most popular citations indexes (Scopus, Web of Science, ERIH) is very low. These search engines are periodical-focused whereas numismatics has many sources outside periodicals (exhibition and auction catalogs, books, conference proceedings). The author estimates the coverage at 2%.

Self-citation in numismatic journals is very high - not necessarily good or bad by itself (although self-citation is sometimes associated with "gaming" the system in order to boost search results), but demonstrates a degree of "self-sufficiency" that may highlight a need for more inter-disciplinary approaches.

The age of quoted literature in numismatics is much higher on average than in the humanities. The author is not especially alarmed and points to the static nature of old auction catalogs, a large corpus of important 19th century literature, and the International Survey of Numismatic Research, which is published only once every six years.