Ignoring for the moment I don't think four years is very "brief" or eight is only a "handful"[1] (and it may actually be even more[2])
It's a bit naive to think this "had no significant impact on the national average salaries" - some of these companies are/were among the largest and most desirable companies in tech, what they do is going to affect the entire industry.
e.g. if you are a hiring manager at a company not involved and are trying to hire someone who is also considering an offer from an involved company the amount you need to offer to be more attractive is less. This brings down the average you pay, which brings down the average you offer. Likewise companies now competing for candidates also receiving offers from you now have an easier time sweetening their offers, and so forth and so on.
[1] https://en.wikipedia.org/wiki/High-Tech_Employee_Antitrust_L...
[2] http://pando.com/2014/03/22/revealed-apple-and-googles-wage-...
If you're considering another offer for X, I can make my offer more attractive by exceeding X. The lower X is the less it cost me to exceed it. If some companies (particularly large and desirable ones) are lowering X that will affect far more than "just" the 64,000 members of the class action.