Solved.
The key thing is that the pinboard feed has a default namespace:
<rdf:RDF xmlns="http://purl.org/rss/1.0/" ....>
rather than just named workspaces, as in the other feed, which worked smoothly:
<rss xmlns:content="http://purl.org/rss/1.0/modules/content/" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:atom="http://www.w3.org/2005/Atom" version="2.0">
so the xpath expression for the feed elements is //default:item and
the field mappings are default:title => post_title, default:link=>guid, and default:description=>post_content
This works as expected.