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I thought the same thing. Becaus interdimensional portals don't just open in another world. It can be open in diferent times, ex: Emma and Julian crossing the portal to Thule moments after Annabel and Ash, but arriving years later. Faerie in itself seems interdimensional. Time dosent flow like anywhere else. What if it's not the land speeding up but the distance between worlds?
What if I told you TLH doesn’t happen in the past and it’s happening around the same time as TDA?
Let me explain. In COG Cordelia saw a “view of dead ground and broken trees, and a red moon rising”. The desert, the mention of a red moon, is literally a description of Thule. I couldn’t understand how it was possible if Thule and our dimension split during the dark war, way after TLH.
But what if TLH isn’t happening in our dimension but another one exaclty the same as our own, except is 1903. Maybe time passes slower, so it’s identical but with different times.
And the Avenger Assemblence Cassandra Clare talked about, with the characters of the shadowhunter chronicals in TWP (And has confirmed they aren’t ghost), is possible because they travel from their dimension to ours, like Emma and Julian did.
Just a thought