Lewis, thanks for asking. I’d meant to dig into this very question myself and had found the same two plugins hiding in the source that you had. Argo links looks like an older project that may not be maintained anymore while the newer CurateWP is a paid plugin.
I’ll ping them and see what sort of response we get about how they’re collecting and presenting them. My reading page is built using David Shanske‘s excellent Post Kinds plugin. I suspect if Nieman is paying a larger fee, they may prefer to do a custom template for a reading page and use something for free.
While I’m thinking about it David may appreciate the structure and functionality of these as I know he’s been thinking about something to replace bookmark related functionality, though I think he has something much more comprehensive in mind.