I am going to provide additional context from the ebook version of Word by Word that I have (and to which I can't provide a link).
Prior to that sentence she says this:
The Atchison Daily Globe shares this bit of breaking news with its readers in 1882: “Parson Twine has a new word—irregardless.”
What’s remarkable about all this is that the word’s earliest uses in print, from the late eighteenth century to the mid-nineteenth century, are unremarkable. There are no scare quotes, no italics, no [sic]s—just the word appearing in print as if it were any other word. But by the end of the nineteenth century, it’s suddenly become evidence of an undernourished mind:
She then quotes three paragraphs of text, the final one being the one given in the question.
In short, it's not her who uses the so-called word inuseless; it's a word that's been used in an 1882 article in The Atchison Daily Globe.
Additionally, she's making it clear in her commentary that some so-called words were not actually defined words at the time, and that there was no stylistic context given to make it clear to readers that they weren't actual words.
In conclusion, inuseless is not a word. It's just something that the author has made up—but also not indicated it's been made up by using scare quotes, italics, or [sic]s "as if it were any other word."
What it's actually supposed to mean is speculative. but it certainly seems likely that it's being used sarcastically and rhetorically in order to make a point about the (then) ungrammatical nature of irregardless.