The term horsepower was coined by James Watt, a Scot, in the late 1700s, so Issac using the term isn't the anachronism is seems.
However 1 horsepower is much less than the actual power of 1 horse, so Issac's carriage is much faster than he said it was.
The guest whiskey taster, Elliot, very much appears to be based on real-life wine and spirits writer Fred Minnick.