Now, perhaps it was just sloppy writing or editing, and what J. L. Bell meant was that the poem was the composite. After having the same reaction as you, and re-reading the sentence a couple of times, I figure that's what was meant. But what a ridiculous trashing of Revere just to point out that Bissell wasn't given his due.
Now I shall take Bell to task for not giving credit to the horses...were the names of any of them mentioned, especially after dying for the cause?! :-)
Well, maybe we are underestimating the perfidy of Paul Revere. Why, he may have bequeathed a sum of money to invent the job of publicist, setting up a trust fund for a poet to write something memorializing him 85 years hence from the night of his ride. How sneaky of Revere to manipulate from his grave the famous poet Longfellow. Then again, perhaps Poor Richard put him up to it.
:)