Reciprocity performance, as well as other emulsion characteristics, appears not directly related to silver content. That's my observation with T-Max, which is reputed to have less silver than conventional-grain films, and the stated position of other posters (such as Ron Mowery) who possess real knowledge of this subject.
Ilford's documentation is obsolete and thin in areas. As a company still getting up to full speed after reorganization following receivership, I'll cut it some slack on that score. Kodak, which had the most extensive and reliable product support documentation in the industry, also suffers from lack of attention to those data sheets now. Kodak reciprocity information is out of date at least since its film production was relocated.
Should you be interested in accurate reciprocity data for many current films, I suggest reviewing work published by Howard Bond in the July/August 2003 issue of Photo Techniques magazine (Voume 24, No. 4). Among the products he tested, Ilford Delta 100 performed exceedingly well, a result not consistent with your allegation that lack of silver degrades this parameter.