I see several issues in this section:

1. The section (in the printed book p. 285) states "Assuming most genes are differentially expressed across individuals, then, if the Poisson model is appropriate, there should be a linear relationship in this plot." It is not explained why this should be so. Is it referring to the mean and standard deviation in the Poisson distribution, both being lambda? But that wasn't covered in the course.
2. The plot which is then generated (in thunk "var_vs_mean") displays variance against means. I think this should be standard deviation? Indeed, plotting sd against means shows a pretty linear picture, with the diagonal cutting through the middle.
3. In the paragraph following the figure it is unclear what the "this" refers to: "The reason for this is that the variability plotted here includes biological variability [...]." Does the "this" refer to linearity or the absence of linearity?
4. That paragraph introduces the concepts of "biological variability" and "sampling variability" as if they had been discussed previously. I don't think they are defined at any earlier point in the book. Also, given the seemingly quite linear relation, does this point still hold water?

PS: thanks for the great course and the book!