Sherlock is a scientist and chemist. He deals in physical facts. Mostly.
When he's deducing, he also dabbles in a LOT of anthropology despite claiming to not understand other humans very much.
Magnussen built his empire on purporting to spread the 'truth'. Journalist rely on sources to confirm things they didn't witness themselves. The facts are filtered through a human consciousness who reports, a human consciousness who writes the article, a HC who edits, and all of the HCs who read the article. That's a lot of POVs for fact to trickle through.
Remember playing telephone as a child?
Sherlock takes exception to Magnussen passing off truth, that is unsubstantiated and shapes the world, as facts.