Classical economists didn't seem to use the term 'inflation' in either sense. I can't find any evidence that 'modern economists' have corrupted the original meaning, like you imply.
The author certainly makes this claim, but fails to provide any evidence.
"It is during this period [the era between the mid-1830s and the
Civil War] that the word inflation begins to emerge in the literature, not in reference to something that happens to prices, but as something that happens to a paper currency.7"
