They found no correlation between the number of cases and temperature, and only a weak association of reduced spread with increased humidity. This is, perhaps, the most comprehensive study to date. There have been numerous previous studies, mostly regional, that do show a negative correlation with virus spread and temperature. The authors suggest this is due partly to lack of rigor in those studies. Also, an expert review of this data (prior to the most recent study) urged caution. They note that the studies showed inconsistent results, and it is difficult to generalize the data to what is likely to happen in the world with COVID-19.
There are basically two factors to consider here. The first is the inherent activity of the virus itself, and to what extent it is affected by temperature and humidity. It is likely that, as with most organisms, there is an ideal temperature range for this virus, and too warm or too cold conditions will limit its ability to thrive. However, it may do just fine in the range of temperatures in which humans live. The second factor, however, is the nature of pandemics themselves. It may be true that the virus is so virulent and contagious (as we would expect for a virus capable of causing a pandemic) that small changes in virulence due to temperature are simply not that meaningful. Also, the fact that this is a novel virus and the world’s population is immunologically naive to it may simply be a superseding factor. The expert review reports: