It'd be nice to actually run the experiment and collect the data before saying "it must be so", but hey, that's me. One could easily see how, things like joint levers, CV capacity, neurological efficiency, tendon lengths and attachments, body mass, could affect the 80% test (to name a few) regardless of fiber composition. As well, you can change the results of an 80% test with work.
Depending on your idea of testing, the genetic test is superior especially because some of the athlete subjects were, swimmers, judoka, and the like. What's the 80% test for 100m freestyle? The genetic test applies across the board, the 80% test may not/does not transfer.
The genetic test, while not absolutely predictive, is at least empirical (As in, it can be repeated as many times as you want under any condition, and the outcome won't change.). Also, it doesn't tell you your propensity for building a certain fiber type, it determines the probability of inferiority/superiority of these explosive fibers.
Also, the $100 price tag isn't grossly undoable (Even, if you know what you're doing, the "homebrew" kit would still cost about $50). Not to mention that unlike the 80% test, this test could have strong implications for the F1 (as in first familial not formula one) generation as well.