To complete this discussion, we've got to know more about how to determine the shape of that brontosaurus-like curve for the probability of mean differences.
I'm not going to give a full derivation of it, but try to explain more or less what's going on.
In fig. 2.7 we plotted the probability density of mean differences. It's got that brontosaurus shape, but we need to know more precisely how to determine it.
First of all, what's its mean? We're assuming that the two groups are statistically identical, so on average, they'll be no difference between their means. What about it's width? Well we used the standard deviation or the variance for quantifying that. This will take some thinking.