Hi Maria,
You bring up a good poing about chi-squared tests vs t-tests. I think the answer may be in Chuck's slides, on slide 16, in which he says that ch-square tests are wrong for this type of analysis because they assume that everyone is independent, and so t-tests are they test of choice. Though there may be other differences in the analysis that may be accounting for the choice of tests. Let's see if others have a good answer!
Maria