I think what people mean is that purported meritocracies aren't often actual meritocracies, since unequal opportunities will give some people's contributions more weight than others.

Unfortunately, rather than explaining how self-proclaimed meritocracies often aren't actually judging on merit, such remarks often get wrapped in remarks about cis white males or whatever (or at least - the ones that make the rounds do), which understandably leads to people getting defensive.

Additionally, as a sibling comment shows, there also appears to be some misleading research, which I think is more likely to happen when arguments like these become as heated as they are.

Thus, I'm not sure whether the argument is true or not. However, I think it's good to separate it from the loud voices unsuccessfully making it, and to try to consider it on its own... merits.