"But it wasn’t until I noticed that Oxford comma that it all suddenly became very clear. That’s not right. That couldn’t be the Wall Street Journal." (From OP)

Does anybody know why an Oxford comma would be a signal that the quote isn't from the WSJ?

I've always found that using an oxford comma ("red, black, and blue" instead of "red, black and blue") is just common sense.

Separation by comma is a concrete indication that the items on the list are separated, which is useful in situations where the last two items could mean something different if not separated ("black and blue" has it's own special connotations, whereas "black, and blue" doesn't).