Because misinformation denying the depth and danger of the pandemic can have negative ramifications, even if posts on a board like this play only a very minor part. People are desperate for comfort and a return to normalcy, and any misinformation that seems to offer that is incredibly enticing - and that can cause people to behave in ways that prolong and deepen the crisis.
Social media - even these boards - offers people a range of information. Some is credible, some is not. But not everyone has the time, or the desire, or the ability, to winnow out the good information from the bad. So countering as much bad information with good, even on a WDW theme park board, is important.