No you did not make a mistake
If you read further into the notes there is a section on duo tone methods which uses the second bath with Potassium bromide.. The colour change happens in the toning not the dev as PE points out.
If you stick with a one dev method you do not need Potassium Bromide.
I do my work both ways, so I have potassium bromide on hand as well it is one of the components for my sepia toning which I do a lot of.