This is an absolutely blatant instance of equivocation.

Here's the sentence from the post:

[believes that benevolence toward humanity might involve forcing human beings to do something violently against their will.]

Assume that "benevolence" in that sentence refers to "benevolence as defined by the AI's code". Okay, then justification of that sentence is straightforward: The fact that the AI does things against the human's wishes provides evidence that the AI believes benevolence-as-defined-by-code to involve that.

Alternatively, assume that "benevolence" there refers to, y'know, actual human benevolence. Then how do you justify that claim? Observed actions are clearly insufficient, because actual human benevolence is not programmed into its code, benevolence-as-defined-by-code is. What makes you think the AI has any opinions about actual human benevolence at all?

You can't have both interpretations.

(As an aside, I do disapprove of Muehlhauser's use of "benevolence" to refer to mere happiness maximisation. "Apparently benevolent motivations" would be a better phrase. If you're going to use it to mean actual human benevolence then you can certainly complain that the FAQ appears to assert that a happiness maximiser can be "benevolent", even though it's clearly not.)