The Machinery of Poetry?

John Stuart Mill in the Westminster Review from 1831:

“It would be a pity that poetry should be an exception to the Great Law of progress that attains in human affairs: and it is not. The machinery of a poem is not less susceptible of improvement than the machinery of a cotton mill; nor is there any better reason why the one should retrograde from the days of Milton than the other from those of Arkwright.”

My only question is this: how does a thinker as subtle as Mill (perhaps I’m giving him too much credit) make such an evidently egregious category mistake? Is it the power of the myth of the machine or is it the power of metaphor? Both I suspect.