I would have had no problem interpreting it as a statement of opinion had you argued something like: “Actually, the coverage of the event in question seems totally reasonable and does not seem to betray any unconscious sexism to me. Perhaps you are mistaken.”
Instead, you argued: “You don’t really believe that the coverage of the event in question betrays unconscious sexism. In reality, you are lying about your beliefs in an attempt to gin up controversy.”
That’s hard to recognize as an expression of opinion, because it is about the contents of someone else’s mind, to which that person has privileged access and to which you do not have any access at all.
Perhaps I would have interpreted it as opinion had you had argued: “Rob has a history of misrepresenting his opinions in order to gin up controversy, for example situations X, Y, and Z where he did so. Therefore, I believe he is doing the same in this instance.” This at least establishes that you are trying to convey information and convince others of the soundness of your perspective – the usual reasons one might wish to express an opinion.
See, I’m interpreting the context in light of my perception of your intent in making your comment. In this case, since you do not seem to be trying to justify your opinion, your statement comes across instead as an accusation of bad faith, potentially an attempt to shame the author into not making posts about unconscious sexism in the media.
Accusations of bad faith can be interpreted as an expression of an opinion of sorts. But in this case, the opinion being expressed is about the contents of Rob’s mind (whether or not his criticism is sincere) rather than the subject under discussion (whether the coverage of the DNC betrays unconscious sexism in the media).
