You are right that not every word uttered by the President of the Church at GC is revelation from God. But it is not technically correct to label that a "logical fallacy". It's more of a bad tradition. There are lots of bad traditions, including: "The prophets are always reliable", "Our feelings are always reliable", "The scriptures are always reliable", "Parents are always reliable", "The mainstream of the Church is always reliable", "The Sunday School manual is always reliable", "Impressions we decide must have been revelation from God are always reliable", etc. (If you are noticing a pattern here, that might have been a deliberate bias on my part. My message is: There is no valid way to avoid using your brain. You need to think about stuff.)