The rule given is a guideline, and commas in general are controlled as much by guideline as by legislation; it is often a question of clarity. I think that, in the sentence you quote, the length and complexity of the two dependent clauses have encouraged the writer to insert the unnecessary comma in order to help the reader sort out the structure.
You might be interested in Gertrude Stein's take on commas:
And what does a comma do, a comma does nothing but
make easy a thing that if you like it enough is easy enough without the
comma. A long complicated sentence should force itself upon you, make
you know yourself knowing it and the comma, well at the most a comma is
a poor period that lets you stop and take a breath but if you want to
take a breath you ought to know yourself that you want to take a
breath. It is not like stopping altogether has something to do with
going on, but taking a breath well you are always taking a breath and
why emphasize one breath rather than another breath. Anyway that is the
way I felt about it and I felt that about it very very strongly. And so
I almost never used a comma. The longer, the more complicated the
sentence the greater the number of the same kinds of words I had
following one after another, the more the very more I had of them the
more I felt the passionate need of their taking care of themselves by
themselves and not helping them, and thereby enfeebling them by putting
in a comma.
So that is the way I felt about punctuation in prose, in poetry it is a
little different but more so …— Gertrude Stein
from Lectures in America |
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