The idea of a meritocracy presumes that everyone starts off and continues through with the same level of access to opportunity, time, and money...
Meritocracy is simply the belief that meritorious people will (either deterministically or probabilistically) rise to the top. It is not the belief that the distribution of merit is fair.
A person who ate lots of lead paint as a child, had Down's syndrome, or some similar tragic circumstance, will have very little programming ability and very little merit. This is unfair. The lack of mentally retarded individuals in open source is not evidence against meritocracy.