I suggest that the question is not whether there have been any cases emanating from WDW. After all, there are documented cases originating from locations where even the strictest precautions are observed.
Instead, the question is whether WDW is more or less safe than elsewhere?
When you consider the number of cases reported at other densely populated areas, I respectfully suggest that, so far, WDW has done well.
WDW's theme parks and hotels are private property with essentially no native residents, allowing Disney to enforce strict rules.
You don't like wearing masks, temperature checks, or social distancing? Disney can ban you.
We have thousands of travelers from around the country intermingling at WDW every day. Despite this, we have had no reports of major outbreaks (knock wood) after more than 3 months of operation.
To date, WDW is the proving ground that mask wearing, temperature checks, social distancing, and regular disinfecting reduce the spread of COVID-19.