I'd like the article better if it cited sources for a number of the facts it claims, like the one about sourdough 'predigesting 97%' of the gluten.

There's little question that sourdough does something that affect the glycemic index of bread, though I'm not sure if they've identified what the agent is or what it does, but I think that would affect the starch, not the gluten proteins. There are a number of journal articles on sourdough, but I've not tried to read more than a couple of them. I'm not sure if anyone at UNL, where my wife works, is doing research on breads at the moment.