Andy,
Yes, your understanding of the /chemicals_of_this_type is correct. It is used to classify "instance of this type" ... and not "part of" parent-child relationships. Here is the actual definition used in Freebase:
Chemical compounds that are included in this classification
However, there are a few assumption errors in the data (Object/Values), that I agree probably need to be corrected later.
Not sure how chemists classify a component of a Thing, i.e., the following statements need to be analyzed and understood by a chemist, but then there are layman concepts that should also probably be held in a new Wikidata property if the community finds it useful to explain chemicals:
"
It is one of the two components of starch, the other being amylose.
"
" Starch branching enzyme introduces 1,6-alpha glycosidic bonds between these chains, creating the branched amylopectin. "