Is it that simple? Yes and no
What Ruiz explains is the truth, but to realize the truth one must transcend all duality and be established in non-dual awareness. My own Master never pays much importance to karma in his own talks, because to focus too much on karma is to continue to remain bound by the illusion that we are trapped in the relative plane, with dualities of pleasure and pain, good and bad. What Ruiz has described is a state we can all attain to by transcending the mind, not by continuing to be a slave to it and our desires, no matter how noble. This does mean surrendering all personal power to what
is, to Existence, or God, or whatever name you wish to call it.
When we are able to accept all that comes our way without judgment or preference, then karma fails to bind us, and we are free. That is the state of bliss that all sages have praised as the
summum bonum, the peak of human realization. The Ocean has ceased to identify itself with the little wave. It does not mean that you will no more encounter effects of karma that has already been put into motion, such as physical disease, but one has attained the eternal realization of oneself as the eternal substratum of all life, and so while the body may feel pain, there is
no more suffering in the mind. However, as long as one remains identified with the relative realm, where desires bind us and cause us to wish things different from as they are in our lives, the Law of Karma strongly holds true.