Unless they retconn it, it can be assumed that Kil'jaeden had the ability to interact with the Shadowlands in some ways. He was able to capture Ner'zuhl's soul, torture it and put it in an armor that he, or his Dreadlords, stole from Maldraxxus along with Frostmourne (which btw is also a retcon of the Frozen Throne).
So many questions about Shadowlands so far, mostly out of inconsistencies with previously established lore and in-game facts. I like the concept of Shadowlands, but at the same time I'm afraid I'll be left disappointed and hating it (just as I did with WoD's time travel AU crap) for all the retcons they'll need to do if they want to make sense of it all. Or they might just leave it unexplained or unfinished like so many potentially awesome lore threads before...