You don’t need to warm up to stretch.
The authors do not believe that you really need to warm up before you stretch. Their example is that unless you are trying to put your foot behind your head as the first thing you do when you wake up in the morning, you will not have to worry about hurting yourself.
I just want to point out that they did qualify this rule. In general, stretching at any random time can be good for you. If I am going to spend an extended amount of time stretching, however, I prefer to warm up a little bit as it makes the stretching a much more pleasant activity and I don’t “pop” my bones as much in the process. As such, most of my concentrated stretching happens after my gym workouts or my runs.