The problem here is that you are appealing to internal consistency that does not agree with your conclusion.
Warcraft has never been a setting where each character type is isolated and cannot borrow power from anywhere else. Mages were already pseudo shamanistic by virtue of two of their three schools being elemental magic types conjured through arcane construction. Mages are also prone to learning from and tapping into any other school of magic they have access to and think would be worthwhile--many mages have had no problems dipping their fingers into Fel magic, or death magic, or void magic. You have modern rogues tapping into shadow/void magic constantly, and historical rogues tapping into spirit magic and shadow magic; you have priests historically tapping into all sorts of secondary magic; druids have from day one also been able to tap into arcane magic via astral spells; warriors have been dabbling in elemental magic since WC3, warlocks have always been masters of not just fel magic, but also raw elemental fire and also void magic, hunters have always used nature magic, arcane magic, dabbled in elemental magic, monks are able to channel elemental magic, spirit magic, and nature magic.
A mage who has gone to the realm of death, been taught how to use anima and death/spirit magic a bit and comes back to Azeroth realizing that the anima method of redirecting energy flow a la Shifting Power is useful and continuing to tap into that learned knowledge is entirely internally consistent. No rules are being bent or broken.