It's not something I'd generalize on. Drawing too much from imagination can lead to all your work being self-referential and repetitive. It can also reinforce bad practices if you're drawing something "wrong" and don't have a reference to check against.
Of course, everyone finds a balance between life reference and imagination as they progress in art. The "draw from life!!!!!!!!!!!" rule the Eatpoo guide sets forward is one that is eventually meant to be broken; while the statement may be flawed, functionally it works: those who follow it will have a strong foundation in realism and nature, which GREATLY reduces the chances that they'll get stuck in an overly stylized trap that will stunt their artistic growth.