IOR for solid materials
I understand what IOR for transparent materials means: Index Of Refraction. But if the "Specular" value determines the IOR for solid materials, I have difficulties imagining a refraction. I associate "Specular" with "Glossy" and understand the value for "Specular" as the strength of that reflection. With the "Clearcoat" it's different. I can imagine this as a transparent glass or plastic layer with some refraction. But you described it in this video as a second mixed in "Glossy Shader" so that there isn't any layer thickness and therefore no IOR.