Interesting that google gave you that hit, tsuwm, since I found the letter on another site. Wonder why google didn't pick up the other site, too?Never mind. I just checked out my research route and ended up at the same place you posted tsuwm. Guess I went around my cyber elbow to get to my cyber thumb.

In reply to:

The Keats letter contains six lines by Keats's acquaintance Horace Smith, which Madden declared "very clever"

So, Horace Smith is the poet in question here.