Can someone explain the differences between this and weak references in a VM with garbage collection?

To be pedantic, scheme is garbage collected, and several use a VM.

PLT already has weak references, and the author assumes the Scheme implementation already has them.

As far as I can tell, this is somewhat like a weakHashMap, except each ephemeron only handles one key value pair.

PLT already has ephemerons too:

http://docs.plt-scheme.org/reference/ephemerons.html

Thanks! That link says it pretty clear: In particular, an ephemeron can be combined with a weak hash table (see Hash Tables) to produce a mapping where the memory manager can reclaim key–value pairs even when the value refers to the key.