Unfortunately there is no way to PROVE anything but if I had to try it would be this.
When they talk of linear timeways think past, present, future. If you had a non-linear time you could spend 50 years in the present building and army/empire/world in a single second of a linear timeway. The real magic of it was choosing the exact spot he wanted to spread his influence. He chose to interfere with the corruption of the Orcs by the Legion.Wrathion says: Kairoz! I want you to meet one of my most accomplished of agents.
Kairoz says: A pleasure.
Wrathion says: Tell us again why this island is so unique?
Kairoz says: We do not know! That is precisely why I am here.
Kairoz says: Time is frozen in this place. The island teems with life, yet somehow never changes.
Kairoz says: Until recently this place was lost in the mists. Those few inhabitants who found themselves stranded here were forced to mint their own currency, immune to time distortion. It is the only way trade is possible.
Wrathion says: I would very much like to study such unique items.
Kairoz says: As would I! The presence of the Timeless Isle is proof that many things are possible beyond the boundaries of linear thinking.
Wrathion says: Oh?
Kairoz says: Indeed. After the Cataclysm, my flight's connection to the timeways forever changed.
Kairoz says: What if it were possible to shape and mold time as you would a ball of clay? What possibilities await? What new worlds could we create?
Wrathion says: ...I like the way you think!
So again the only way it makes sense to me is to think of our timeline as the X plane on a cartesian graph. From 1 to infinity is past to future.
Garrosh's Iron Horde's timeline is the Y plane, *almost perpendicular to the X plane neither moving towards the past or future but progressing in its own future from 1 to infinity apart from the timeline of the X plane (Our time).
This is how he created Legions of Orcs with advanced weaponry in two short months in our time. He stopped time, chose his point in history and created a brand new world from this moment in time. Any planet like Azeroth (X planes) would only have seconds to prepare where they have an entire lifetime (Y plane). A truly endless army. Thus the Iron Horde is from a Perpendicular Universe suspended between past and present just like the Timeless Isle.
Accept this of simply look to the picture below.