Roots and callus both come from the cambium layer but they are not the same. I love the theory because it makes sense. At this site http://www.saps.org.uk/saps-associates/browse-q-and-a/500-what-causes-root-callusing-why-is-it-that-some-in-some-varieties-some-of-the-cuttings-root-readily-while-others-develop-a-large-amount-of-callus-which-prohibits-rooting this is what is said “Root callusing in cuttings
What causes root callusing? Why is it that some in some varieties some of the cuttings root readily while others develop a large amount of callus which prohibits rooting.
Incitement for root production is initiated as soon as a stem is severed from its parent. This is called wound shock.
The first reaction is the formation of callus tissue which protects the wound. Next, roots are produced.
Both the callus tissue and the roots arise from the cambium (layer of actively dividing cells found beneath the bark).
The formation of callus is a necessary preliminary to rooting, although roots do not arise in the callus, but from the cambium immediately behind it.
Rooting is encouraged by moisture, warmth (but the air temperature should be below the that of the soil), good aeration.
Hormones (especially synthetic auxins like IAA, IBA and NAA) assist in the formation of roots, especially when the soil temperature is at least 15ºC.
We guess that the initiation of roots in different cuttings is hormonally controlled and experiments with different hormones and/or different concentrations would be a suitable topic for investigation.“
I do however believe scions unable to root are unable to callus as mentioned in this article https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4001394/#!po=15.0000