If you feel that they're talking about a problem someone else solved, ask them that directly. (Did you work with others? etc) If they're lacking on the technical details either it's been a long time ago or they didn't do it.
I would think they and their colleagues discussed alternatives together, collaboratively in for example a Slack chat — so an interviewee can give you good replies about the thought process and alternative solutions that were considered and discarded. I would assume. Or maybe the interviewee him/herself came up with ideas, that his/her colleagues realized weren't going to work, and explained why, for him/her. Then s/he might be really good at describing the thought process.