Perhaps there's some unmeasured influence, but this study was looking only at the difference between growth within subjects vs between subjects. If the subjects were all "newbies", then that doesn't explain the results.
They're essentially saying that individual genetics explain the majority of the variation seen as a response to muscle stimulus in their test subjects, not the mass used, because the variation within the test cohorts was greater than the variation between them. You can argue that, if they didn't test experienced lifters the results might be different in that population, but you can't dismiss the results on those grounds.
They're essentially saying that individual genetics explain the majority of the variation seen as a response to muscle stimulus in their test subjects, not the mass used, because the variation within the test cohorts was greater than the variation between them. You can argue that, if they didn't test experienced lifters the results might be different in that population, but you can't dismiss the results on those grounds.