From the article: "A total of 32 women and men completed this oat-based diet. They ate 300 grams of oatmeal on each of the two days and only consumed around half of their normal calories."

"The positive effects of the oat-based diet tended to still be evident six weeks later. "A short-term oat-based diet at regular intervals could be a well-tolerated way to keep the cholesterol level within the normal range and prevent diabetes," says Junior Professor Simon. However, in the current study, the cereal above all exerted its effect at a high concentration and in conjunction with a calorie reduction: A six-week diet, in which the participants consumed 80 grams of oats per day, without any other restrictions, achieved small effects. "As a next step, it can now be clarified whether an intensive oat-based diet repeated every six weeks actually has a permanently preventative effect," continues Simon."

So, what I am reading: 

1) the purpose of the study was to evaluate cholesterol levels, not necessarily to evaluate blood glucose levels

2) the participants were eating a reduced calorie diet.  So, if they were consuming a regular calorie diet, which would include oatmeal, would they have seen the same level of results?  I suspect not.  

The oatmeal has benefits from the fiber.  So, the combination of having a healthy dose of fiber, along with the reduced calorie intake, is what is causing the benefit.  At least, that is how I read it.