I think this is absolutely true. I recommend that everyone reads "What sport tells us about life" by Ed Smith. It is an excellent book and one of the chapters covers the economics of sports wages. In it he reviews some of the work done in comparing the competitiveness of sports in relation to their labour policies.
The key to the competitive balance story, concludes The Wages of Wins, is not league policy, but simply changes in the population of athletes the league draws upon.
I think the real story of Wigan's dominance of the 80s is that the player pool in British RL had become so denuded that there simply weren't enough high quality players to fill the teams playing at the top. Those that were good moved to Wigan partly because of the pay, but also because Wigan was the place to win things and good athletes are motivated by silverware, not pieces of silver. It became self-fulfilling, Wigan won things so you had to move to Wigan to win things. It was a waste of time playing with the second-raters who populated all the other teams.
I think in the light of the economic studies into wage policies you have to conclude that the salary cap isn't a major factor driving a levelling out of the game. If it's occurring at all it's more likely to be related to the huge increase in playing numbers in the community game and the massive improvements in the systems that bring through elite young players.
That doesn't mean the salary cap is worthless. Far from it. We still have a small player pool to choose from and it seems that the evidence shows that where this is the case labour restrictions can improve the competitiveness of a league. So until such time as every club in the league is able to field a predominantly "home grown" side it's probably worth sticking with for its levelling effect, limited though it is.
The second reason for having the salary cap I think is a much stronger one. Quite simply it stops clubs from overspending. Regardless of how arcane the rules are and even if some clubs are finding loopholes the general principle prevents an outbreak of the kind of spiralling wage inflation that could lead to massive financial damage to a number of clubs, big and small.
Lastly, the evidence presented in studies on labour controls within sports leagues clearly shows the importance of widening the available player pool. It seems obvious when it is said, but it nonetheless needs to be said clearly and often, the health of the game depends almost solely on the ability the game has to attract young players and successfully develop them into elite athletes. The importance of that simply cannot be understated.