a major weakness of his argument is that he implies (credibly) that this lab test commissioned by Apple is not trustworthy, and then compares it with tests by Dell and Intel, which he seems to present as implicitly true

He only mentions the results that Dell and Intel produced for Dell and Intel platforms (all the Apple results are the ones given by Apple). In other words, if Apple wants to be biased in favor of G5, so should Dell/Intel and that is exactlly what he presents. It sounds like a valid argument to me!