I also remember reading this - that unless there is oil and grease which requires soap to remove, then water alone is effective. At the time it seemed to me it was somewhere fairly reputable. From memory, the idea was that soap reduced the friction, and actually might lessen the effectiveness. I think it wasn't comparing best-practice handwashing with and without soap, but focused instead on the kind of handwashing that people actually do.

Hard to find anything on it now, and I understand soap was particularly effective against SARS-CoV-2 compared to water alone (can I just call it the COVID-19 virus?) so it looks like the idea is mostly gone now.