you must not reveal that you think that
I think the actual demand is somewhat narrower than this, probably? Or at least, my instinct in a situation like this is often to be careful about the manner in which I discuss my thoughts, rather than to outright conceal them.
Someone could plausibly object even to the narrower requirement about the manner in which one reveals that, but I do think it’s narrower.
The obvious thing to me to try would be italicizing the host