I love the Oxford comma, because it leaves no room for ambiguity. I've heard anecdotes of people omitting the comma in contracts and having it come back to bite them in the butt... but who knows if there's any truth in them or not.
Frankly, with writing in natural languages, hard-and-fast rules like, "Oxford Comma are always unambiguous," and, "Always avoid passives," tend to make your writing weaker, because there are still some cases in which Oxford Commas are ambiguous and passives are the right tool for the job. What makes great writing, whether it be creative writing or contract writing or what-have-you, is knowing when to apply such rules and when to disregard them.
In this case the serial comma isn't creating any ambiguity, the bad writing is (and the serial comma shifts the ambiguity to the second/third item being listed). When they suggest a better way to write the example sentence, it uses the Oxford comma both times (though I'd argue that the second sentence isn't a solution).
I agree that nothing is ever absolute (hah), but the Oxford comma takes care of ambiguities most of the time, and merely shifts the ambiguity around when appositive phrases are thrown into the mix. And reducing ambiguity should be the goal of every writer.
The thing about just admitting you can rewrite a sentence to be un/less ambiguous while keeping the comma is that it simultaneously admits that you can rewrite a sentence to be un/less ambiguous without the comma... which puts us back at aesthetic arguments.
My favourite example of this is "We partied with the strippers, Stalin and Hitler".
> Without a serial comma, the above dedication would read: To my mother, Ayn Rand and God, a phrase ambiguous only if the reader accepts the interpretation my mother, who is both Ayn Rand and God.
That's exactly the same grammatical interpretation (i.e. the comma indicates an appositive) that was suggested to be ambiguous with the Oxford comma present.
The ambiguity in the example is in whether his mother is Ayn Rand, not whether she's god.