The problem with the code is it makes no allowance for distance from the clay liner to the outer masonry, or mortar mass between the liner and the masonry.
You could have a 6 foot wide chimney with one clay liner down the middle, like mine, but if the sides of the masonry touch the house (like mine) it's not code. That just does not make sense to me.
Covered by one of the three exceptions contained in IRC R1001.15.
"3. Exposed combustible trim and the edges of sheathing materials, such as wood siding, shall be permitted to abut the masonry chimney side walls, in accordance with FIGURE R1001.15, provided such combustible trim or sheathing is a minimum of 12 inches (306 mm) from the inside surface of the nearest flue lining. Combustible material and trim shall not overlap the corners of the chimney by more than 1 inch (25 mm)"