org-ref should not get any credit for this. That is a feature of the built in org-cite library, which uses completing-read for that behavior. completing-read is only for reading one thing at a time, and org-cite works around that in a way that leads to the behavior you see.
It looks like helm (if you use helm-mode), but it is actually the helm version of completing read, and not a helm selection command.
Other completion backends use different bindings, e.g. ivy uses C-M-j I think.
John
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Professor John Kitchin (he/him/his)
Doherty Hall A207F
Department of Chemical Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
412-268-7803