It's a common misconception that the paleo diet means no carbs. It's just the type of carbs that they prescribe or proscribe. All plant matter is predominantly carbs and is where most of the carbs in our diet should come from. Breads, donuts, pancakes, sodas, etc are highly refined carbs that bear little resemblance to the carbs our paleolithic ancestors ate. On top of that, there is growing evidence that a grain based diet is highly detrimental to our health.
Regarding cattail flour, there are several things to note. One, it was coarsely ground which is in direct contrast to the majority of flours that we currently consume. Whole grains are in general better than overly processed grains if you're going to eat grains. That being said, the evidence for our ancestors eating cattail flour comes in around 30,000 years ago. The Paleolithic period goes back 2.5 million years and the evolutionary history of our digestive habits is largely without any grains. Just because we have been eating grains for 30,000 years does not mean that's what we ate throughout the majority of our evolutionary history.
I'm not a nutritionist but I do eat a predominantly lacto paleo diet and the changes in my body fat, sleep habits and energy levels have been extraordinary. Anecdotal? Yup but it can't hurt to give it a try. That's usually my advice on the paleo diet. Far from being a fad diet, it's much more like a lifestyle change that seems to make sense in my experience.