Perhaps because "Generation" continues the Parent/Child allegorical language.

Think they're using "generation" here in the context of number of parents (yes, "depth" would work fine as well, but is a more general term)

  > number of parents

s/parents/ancestors/

But that statement only holds up on branches without any merge commits. Because the actual algorithm does not just total up the number of ancestors.

Since gitk and others put new commits on top, i'd propose "height" instead of "depth" ;)