Most organizations, and certainly Baseball America, rank prospects on the likelihood of being impact players.
I think their point was that the Yankees have a large quantity of players who might make an impact, the quality of that impact from many of them might not be so great. A future utility infielder, for example, is a future major leaguer. While this is a good thing to have, it doesn't really increase the impact from the farm substantially.
But it does allow for one more minimum wage player on the team, increasing the available budget. However, this is likely not taken into consideration when measuring the impact of the farm, and it should not be.