It is highly unlikely that Disney would refuse to accommodate a hotel guest who thinks they may be infected by Covid. There are hospitality laws designed to protect guests from unscrupulous hoteliers. Generally speaking, when a hotel accepts a reservation, they are obligated to provide a room. Period. You cannot refuse to allow someone to check in for arbitrary reasons, nor can you (easily) evict someone against their will.
There are provisions in these rules for public safety. However we have no guidance from CDC, WHO or similar organizations stating that hotels, apartments or similar close quarters living is unsafe. If CDC does not believe hotels are unsafe during the Covid pandemic--knowing that the presence of contagious guests is unavoidable--then Disney is on shaky ground making that call on their own.
If a guest admits they may be contagious, it would be wise for Disney to do their best to accommodate. They have every right to charge for an extended stay or other services provided. (But they may choose to not do so.)
In the 1.5 months since the resorts reopened, there HAVE been Covid-positive guests. It's unavoidable. If you choose to visit now, treat everyone like a possible Covid carrier. Wipe down your room upon arrival if you wish. Wear masks. Avoid physical contact with others. Sanitize hands frequently.