Don't think you've missed anything. Historically I think the shell conduction model linked the layers, and didn't account for the thickness when working out where the heat went. There used to be some slides somewhere but these may be an old Fluent (pre Ansys) User Meeting: I can't remember. So in the above the inner "green" layer can be nearly zero thickness of something and the black layer will then link into the blue part.
Please test this: I'm working on memory, and it's turning into a long week....
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