Miscellaneous must be followed by a plural count noun; it does not work with an abstract mass noun. Exceptions are set phrases such as miscellaneous shower/income.
An abstract noun refers to something that has no physical existence; sometimes the meaning of an abstract noun changes from singular to plural <kindness–kindnesses>. Some mass nouns can also be count nouns on occasion <choose only the best meats>.
According to Wiktionary it comes from the Latin nominative masculine singular of miscellāneus.
- With a sing. n.: Consisting of members or elements of different kinds; of mixed composition or character. With a pl. n.: Of various kinds.
What is Garner's statement based on then?
Secondly, regarding the definition of abstract noun, I don't understand what the pair kindness(es) is referring to.