What you quoted was the second part of two related sentences. You can think of it like an if / then statement. I will write it out again here with the IF and THEN listed explicitly:

IF you are riding at a constant power output/heart rate,
THEN drafting means you can ride faster at the same power output/heart rate.

Nowhere in my post did I state that you should ride at a constant power output/heart rate all the time.

If you want to get a better workout at a given speed, then don't draft.

This, however, is not what I was talking about.