p. 177 right below the materials section. It says steel shields have a hardness of 5 so it looks like light vs heavy changes the AC bonus but not the hardness.
HARDNESS
The table below provides the Hardness for some types of
common materials. Use the thin Hardness for shields and
thin items and the regular Hardness for sturdier objects
such as armor.
Yah that's a good point. I wish they included the thin item/shield hardness on that hardness table on p. 354 since it is liable to lead to confusion as is.