More likely it's because 6 months have elapsed. Cellular immunity to SARS-COV-1 (2003 outbreak) is still robust 17 years later - the two coronaviruses are fairly similar.

People keep parroting that article. "Cellular immunity" mentioned in it may be not enough at all to prevent reinfection. Unless there is a direct rechallenge with the SARS-1 virus someone who already had it in 2003, there is no way to make such a strong statement. Good luck finding volunteers.

It's amusing how you put "cellular immunity" in scare quotes. It's not a new or novel concept, but an extremely well understood mechanism that has been taught to immunology first years for decades. That a coronavirus should exhibit a cellular immune response should be the baseline assumption, rather than the inverse.

From the article, it seems like the latter. They tested at 6 months. If they test those same people again later, then we will possibly have a new high-water mark for the duration of immunity. For now, we can only say “at least 6 months”