First Covid-19 reinfection documented in Hong Kong, researchers say (statnews.com)

Good news: the person in question was infected by a different strain of the virus and his immune response seemed to prevent more serious symptoms from manifesting.

Bad news: herd immunity from natural infection seems unlikely and would only achievable with a vaccine.

Does anyone know if they only tested for RNA, or for live virus?

At this point in time it is absolutely unacceptable to assume that presence of viral RNA equates to infectious virus. Hence the question.