Purely conjecture on my part but the irony of Sam's familiar use of the
phrase "mark twain" in juxtaposition with its use in Vanity Fair would
be too much for him to ignore.  Particularly someone with his sense of
humor.  I can almost imagine the flash in his mind,  the potential for
hoax(es). Right away he started pretending it originated with Capt.
Sellers.

I wish I could afford subscriptions to journals...