The plot hook thing is an extra challenge, so not necessary!
Really, what it's asking is to provide 3 (or more) ideas that GMs can use to spring off into their own games or campaigns. How you include them in the setting alongside how much detail is up to you! For example, I could have a section in the setting that is titled Plot Hooks with bullet points like "Grandma Sherry says that her cat went missing, but was last seen at Henley Lake." Or, you can weave it into the actual writing of the setting.