Plaguescarred
Hero
I agree with you. Arial Black is taking the sentence out of context. The threat that surprise creature do not notice is in reference to anyone hiding that the DM compared Stealth vs Perception against. There is otherwise no surprise if no one is hiding as they automatically notice each other.I disagree with this, although it does appear to be a popular approach. To me, a straight reading of the rules indicates that you don't need to recognize the creature as a threat. You only need to notice the creature and you will not be surprised by it.
The rules tell the DM how to determine surprise. Someone has to try to be stealthy. If they do, their Stealth check is contested by the Perception check of anyone focused on detecting hidden threats. That's it.
Surprise: If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side. Any character or monster that doesn’t notice a threat is surprised at the start of the encounter.