Law in Singapore is different than in the US. But regardless, it is fully legal to exclude people based on a fever.
In fact -- legally speaking, as a lawyer myself, I would recommend Disney perform temperature checks.
Why??
Imagine Disney re-opens.... and a mini-pandemic breaks out at Disney, hundreds of guests get infected, dozens of guests die..
Many families SUE Disney.
If I'm the lawyer for the families that are suing, I say: Disney acting negligently. They failed to take adequate safety precautions to protect the public. Theme parks through Asia required temperature checks, etc, to reduce infection. Disney did not take reasonable steps to protect the public.
So if I'm the lawyer for Disney right now, giving them advice, I would be telling them, "you are facing legal exposure if people get sick at your park, and you didn't take reasonable steps to lower the risk of sickness. If parks in the rest of the world are checking temperatures and you don't do it, then you can be found negligent."