If you think that a talk on Tu B’Shvat (which the Rayim Ahuvim website currently says is the topic) will contain kefirah and will therefore also be lifnei iver, that’s your right. I guess no-one will really know until they hear the speech.
With regard to “legitimization,” I don’t know. Is anything more than a speech legitimized when a speaker is given a podium? When politicians speak at yeshiva dinners, are the yeshivas endorsing every value and action of the politician?