Because it very clearly is? We quite clearly saw that things did happen sequentially according to Time. The Brokers who found Zereth Mortis were known beforehand, so it isnt like it was something that would happen in the future relative to our perception. And similarly the Enlightened Brokers had a clear society built around cause and effect as observed in the past.
Time might have been different compared to how it would be on Azeroth, but Zereth Mortis was still very clearly bound by the same linear flow of time.