This is more confirmation that getting it once doesn’t necessarily infer immunity...or...that there are different strains that have already developed.
That bodes very ill for the prospects of a vaccine.
It seems highly unlikely that she was reinfected, and she isn’t the first to “relapse”. Most likely, her body was able to fight off the virus in her lungs enough to produce negative tests, but it was still present in her organs or some other tissue not being examined. Kinda like the herpes zoster virus that causes Chickenpox hides in your spine, only to come back many decades later to inflict Shingles on its victims. Unfortunately, COVID-19 doesn’t wait.
The same problem might apply to those “cured” by anti-viral drugs. Maybe like with HIV, once you stop the drugs, the virus returns in detectable levels and you go from NEG back to POS. That might explain why China is pushing so hard for exclusive right to remdesivir.