misleading thread
As a prediabetic, one Quest bar only produces about a 10 mg/dl increase in BG, and it takes quite a while (over 1 hour) for that to occur. Other protein bars with more net carbs create much larger increases. (Also note that apparently the mother of the owner of the company is diabetic, and they have tested the bars on her.)
Note first that the much of the protein in the bar is of the type (unlike that in nuts or meat, for example) that is quickly digestible, meaning the it passes quickly through one's stomach and will have a big effect on BG if one is highly insulin resistant and prone to gluconeogenesis (conversion of protein to glucose). This is true for most diabetics, but not for most prediabetics, like myself. Also note that, contrary to what is shown in the video, one needs to account for protein in food when calculating insulin needs (see Bernstein's discussion).
Second, note that not all fiber is alike, and it is likely that some of the "fiber" in the bar is in fact digestible. The combination then of some carbs from the "fiber", plus a lot of easily digestible protein, plus subjects that have high insulin resistance (note the high starting BG of many of those reporting - often an indication of a high rate of gluconeogenesis), makes it not that surprising to see a large BG jump in some people after consumption of a Quest bar.
So, it is probably misleading to put the blame on "fiber", although certainly true that you have to test these things to determine their impact, and can't just rely on net carbs.