Every community has a long list of etiquettes, rules and shared knowledge that is assumed and generally not spelled out explicitly. One of the core assumptions of the rationalist community is that every statement has uncertainty unless you explicitly spell out that you are certain! This came about as a matter of practicality, as it would be inconvenient to preempt every other sentence with "I'm uncertain about this". Many discussions you will see have the flavor of "strong opinions, lightly held" for this reason.