"9.2 plurals: Plural unit names are used when they are required by the rules of English grammar."

This is straightforward.

"9.3 Spelling unit names with prefixes"

This defines "centimetre" as a unit name that has a prefix. It doesn't fall into a distinct "prefix-unit" class. Therefore it is treated as per 9.2.

It seems clear enough to me, George, but I know you don't like to be wrong, so let's see how you wriggle. ;-p