I think it goes beyond prescriptivism/descriptivism.
Descriptivism refers generally to describing how a community uses language, so the most common usages end up being the primary definitions. Once documented, these tend to become prescriptive.
In that context, a single author who uses a word in an unusual way would likely not have any impact on the descriptive or prescriptive definitions of that word, unless of course their usage becomes common.
Humpty was arguing for the benefits of using words in unusual ways, which potentially violates both prescriptivism and descriptivism.
Impressionistic use of words is one example of this, where the words used might convey a certain feeling, perhaps via their connotations, even though their literal definitions may not be exactly appropriate. This kind of usage is generally found in literature where language is being used as an artistic tool rather than a prosaic tool of communication.
