Isn't this comma erroneous? Otherwise, what "type" of comma is it?

So I'm going through The Elements of Style Workbook, and I'm on a section where they give us a quote from Ralph Waldo Emerson written as follows:

What must I do, is all that concerns me, not what the people think.

My question is just this: isn't the first comma erroneous? It would make sense as a parenthetical comma if you reversed the last two clauses:

What must I do, not what people think, is all the concerns me.

But as it stands that common seems wrong and throws me off. If it's not erroneous, what is this type of comma called? Or is just expressing the way he spoke it, or perhaps an artifact from an outdated writing style?

Superfluous in modern usage. Not correct now but was fine then.