edit: Ok, just seen your edit. In the between time 
I did find that exact file. It does open in Gimp 2.10.18 (at least it does here). It is all in one layer group. One difference between Gimp 2.8 and Gimp 2.10 is Gimp 2.10 opens with the layer group closed. You need to expand it. Easy to miss.
As: https://i.imgur.com/WOAZ6nH.mp4
I did find that exact file. It does open in Gimp 2.10.18 (at least it does here). It is all in one layer group. One difference between Gimp 2.8 and Gimp 2.10 is Gimp 2.10 opens with the layer group closed. You need to expand it. Easy to miss.
As: https://i.imgur.com/WOAZ6nH.mp4