A few quick responses:

• This data record starts in 1980 because that is as far back as some of the data sources used in the analysis go. To make sure the dataset in internally consistent, the team can only go as far back as the shortest records.
• This dataset was not developed to track climate change or to prove that climate change is affecting extreme events. It was developed to allow people (insurers, e.g.) to compare the costs of big disasters from year to year in a consistent way. We would expect to see an increasing trend in these costs even if climate were not changing as the value of property in hazard-prone areas increases. However, because we know from other research that human-caused climate change is increasing the severity and frequency of some extremes, we must acknowledge that it is likely a contributor to some of the trends in these data.
• These data are freely available from NCEI and any one interested in exploring them and using them to publish their own research is welcome to do so!