Not tested and under reported are two very different things. The antibody testing and research from virus detectives is showing that the virus was in the United States (including Florida) much earlier than thought. Asymptomatic or people with very mild symptoms would have had absolutely no reason to get tested back when there was supposedly no human to human transmission and all cases were supposedly around Wuhan, China.
Even now, I doubt that anywhere close to 100% of people with a mild cough or slight fever will go get tested.
As far as the parks opening, that has more to do with what type of procedures for social distancing and other preventative measures can be implemented within WDW than what the surrounding communities do. Honestly, neither Orange or Osceola counties really had much of an outbreak to begin with. Both could have been in Phase 1 (or Phase 2) since the beginning.