Is it dehumanizing to report that someone else made a dehumanizing statement without endorsing the statement? I don't think so.

You are essentially asking “will Twitter make use/mention distinctions in enforcing this policy”. While it is of course impossible to state for certain in advance, I don't see any particular reason to believe they would either be disinclined to or incapable of doing so.

My guess is it will depend on whether or not the journalist is considered part of the mainstream media.