I think this advice is out of date. I recall hearing this in the early days of cat converters when most cars had carburetors and idle mixture was set somewhat rich to get a smooth idle. In any case fuel mixture was comparatively imprecise. Excessive idling could allow converter temp to climb to the point where the substrate could melt, ruining the converter. Also, at idle, there's no airflow to cool the converter. As others have said below, modern EFI systems maintain the proper air/fuel ratio for proper combustion and converter operation, even at idle.