Notice that there are fewer and broad box pleats.
Hard tartan is a term that refer to the old method of weaving cloth that left a coarse feel to the material. It was partially due to the fact that the cloth was not finished as modern cloth is and partly a result of the type of fleece used for the yarn. Often this cloth had the appearance of starched linen. It is very difficult to replicate today because the breed of sheep is extinct/bred out and all commercial cloth is now washed and softened during the finishing process, something not done to C18th and early C19th cloth.