Where did you see someone claiming that siping can enhance fuel mileage? I always understood that it would decrease it slightly.
Siping is generally associated with the search for better snow & ice traction, and I have seen reports of testing that shows that it does that. (Here is a very brief snippet that says it increases ice/snow traction, but also increases stopping distance in wet/dry conditions: http://www.consumerreports.org/cro/2012/12/should-you-slash-your-tires/index.htm)
I don’t go by the notion that one can’t improve on the engineers and manufacturers given specific purposes. In engineering and manufacturing there are lots and lots of things that have to be maximized, and plenty of tradeoffs. All possible things are not maximized at once. So if one has the very specific purpose of increasing ice/snow traction siping will probably help, but possibly at the expense of other things - like fuel economy, though I couldn’t say whether the effects would be measurable. I can say that it wouldn’t increase fuel economy.