Its a good question. Arishem does seem to be just good old telepathy, nothing too extra-dimensional. The Duat seems like an alternate reality where time on Earth kinda just stops. I don't know I didn't watched the show, but that was the impression I got from the plot.
Time continued in the Duat. Remember that while there, they saw the souls that were being judged, which didn't happen until the road block, and then the release.
You also need to account for Layla breaking Khonshu free, having an exchange where he explicitly says Marc is still dead, and then the rescue in the Duat that leads to Khonshu knowing Marc is alive again.
