Partially true.
Fuel prices are rising, and Southwest is feeling the pinch.
However, Southwest still has the lowest CASM ex-fuel in the business. In other words, if you were to take fuel out of the equation for all the airlines' CASM, Southwest would still have the lowest costs. So if all airlines were paying the same price for fuel, and there were no such thing as hedging, SWA would still have the lowest CASM. And of course, in a commodities market such as the airline industry has become, the producer with the lowest costs has the best advantage - they set the prices for a given market.
So while Southwest definitely has some challenges right now, and things certainly aren't as easy as they once were, I think people who say, "They'll lose their pricing power once those hedges run out" are not looking at the bigger picture.
Just a thought...