Any new information about reality, if properly understood... can only cause people to become more ethical

Whether this is true depends on your definition of "ethical." In any case, your claim here doesn't weigh against the idea "that certain claims about the nature of reality could cause people to become more immoral" because people do not, in fact, always "properly understand" new information about reality.

Eliezer did say "Doing worse with more knowledge means you are doing something very wrong," but check what he said in the very next paragraph: "On the other hand, if you are only half-a-rationalist, you can easily do worse with more knowledge." The trouble is that current people are indeed only half-rational, or worse.

A particular truth can only hurt someone if he holds a false belief.

A counterexample: Suppose that a human-level AI, Ralph, holds only true beliefs. But Ralph doesn't yet know that Petunia exists. The superintelligent Omega tortures everyone who knows that Petunia exists. Now, Ralph learns that Petunia exists. But this truth hurts him, even though he doesn't hold a false belief.