From the poem:
The poem was typed on a manual typewriter. It begins with four lines on the revolver in question. The last two are: "It delivers unmistakable ultimatums. It is the last word." snip

"George Hendrick is an English professor emeritus at Illinois who edited several volumes of Sandburg's poems. He said the poem is authentic and might have been inspired by President Lincoln's assassination." snip"

Lincoln was killed with a Derringer which Sandburg as a historian would know.

I can only assume the English professor (Hendrick, not Gullerud) is an idiot...

He discarded, or THOUGHT he discarded something that was inaccurate. Somehow someone born to be a poet well after Lincoln was killed is questioned for something he never published?

The type of gun, he found out later, or that day when he wrote this, not what he wrote in poemic style? So the problem is...?

he is best known for his biography OF Lincoln and is even better known for his poetry and the aptness of the metaphors in his poems. He knew exactly how and with what Lincoln was Killed. I don't question the poem's existence, just the interpretation of it's meaning.