I don't know why anyone ever proposes to solve overpopulation problems with this method. As your example numbers illustrate, it does seem rather infeasible. However, I dislike when this argument is used as a means of refuting the possibility and benefits of any and all off-world colonization attempts (it isn't evident if this is the stance of the parent or not, so forgive me if I'm being too critical). The summary of this attitude seems to be "if we can't fix everything in one fell swoop, why bother trying to leave this rock at all?" The potential benefits of space colonisation have been thoroughly discussed on Slashdot, including exploitation of new resources, spurring the development of new technologies, and the concept of an off-site backup for our DNA and our species.
To get back on topic, it is irrelevant if shipping "excess" humans off-world can solve overpopulation. What is relevant is that the mechanism of exponential growth could potentially allow an entire galaxy to be settled in a (relatively) timely fashion. The result may not be pretty, with a multitude of overpopulated worlds, but it is possible nonetheless.