Darwin's "Historical Sketch"

I'm a little perplexed. A line in the "Historical Sketch" pretty clearly identifies it as having been written (at least that line) after 1859:

In 1831 Mr. Patrick Matthew published his work on 'Naval Timber and Arboriculture,' in which he gives precisely the same view on the origin of species as that (presently to be alluded to) propounded by Mr. Wallace and myself in the 'Linnean Journal,' and as that enlarged in the present volume. Unfortunately the view was given by Mr. Matthew very briefly in scattered passages in an appendix to a work on a different subject, so that it remained unnoticed until Mr. Matthew himself drew attention to it in the 'Gardener's Chronicle,' on April 7th, 1860.

Does Johnson argue that last sentence is a later insertion?

Since the paper isn't published yet, I can't definitively comment. However, the article I link to states:

The first published version of the preface credited Charles' uncle, Erasmus Darwin; French naturalist Jean-Baptiste Lamarck; and 12 others. From the first appearance of the preface in 1860 to the final edition during his lifetime, Darwin made several changes and expanded the list of his intellectual debts to include 36 people.

So, I imagine that the reference to Matthews was not in any 1856 version and instead only appeared after 1860.

If Johnson is calling Erasmus Darwin Charles' uncle, there's a problem right there...

John,

I think that is the journalist. I'd imagine the referees for JHB would have picked up that gross an error if it was in the manuscript.