Base 6 Flat Roll is just a suggestion for the GM. But you are right for PCs. NPCs and Monsters do not follow the same rules as PCs, and usually do not have skills listed. So, as the GM, you have to make a ruling at the table(or before during prep) on whether or not a given NPC/Monster possesses that ‘skill’ and what their rank would be.
In this particular example though, Seth is talking about DEFENSE for the PCs, which is NOT a skill, it is a derived value that has a minimum of 6, and rises with Armor worn. However, Seth is wrong about EVADE… once a skill is crossed off due to Attrition, it is not usable until it is healed. The narrative should support this. For instance - PC has EVADE of 9 as a skill and gets hit for Flesh Attrition. The random roll comes up on the EVADE skill slot. The player narrates that the beast just bit his PCs leg so hard it fractured, and now the character is not able to EVADE or move quickly as a result. Narrative leads to the skill being crossed off.