[tinalumfoil]

In a true meritocracy, all candidates are judged on equal footing so P(x|a) isn't compared to P(y|b). In practice this is very possible (albeit expensive) to achieve, given a set pool of candidates. Although judging based on past achievement has the problem you describe, it can be used in place of a fair test (eg, judging candidates based on resumes instead of technical interviews) since high past achievement is strongly (but not perfectly) associated with high skill.

Mozilla doesn't appear to be addressing your version of meritocracy, but the actual definition of meritocracy, which still gives those who had the opportunity to build skill an advantage.