But I think he's confusing two separate questions here:
1.) Do TFA corps members remain in the classroom?
2.) Do TFA corps members remain engaged in education?
It's a fact that most do not teach long, if at all, past their two year commitment. That's not necessarily a bad thing, though it does make it impossible for TFA in its current form to be a large-scale solution for the teacher quality problem.
But I've always argued that the second question is more important. There's a bevy of anecdotal evidence that TFA members move on to other careers (both within and outside of education) that impact our educational system. We're already seeing TFA alums serving as principals and superintendents, and in the next decade I think we'll see a growing number of TFA alums taking leadership roles in academia, government, and other fields as well. To me, this will be the largest impact of TFA in the long run.
So, regarding his post, I don't think a large turnout would say anything about the first debate, but it would say an awful lot about the second.