Exactly. There was no substance behind the mystery that Hadrios presented, just the writer's attempt to shake up the event and make it more interesting than it already was, which it didn't need to be. The Fall of Sotha doesn't need a mystery attached to it, it's already an awesome and tragic event that has enough weight to it that it could be told straight with no additions to it at all and it would still be compelling just to read such an epic and doomed defence against such an incredible nightmare that is a Tyranid invasion, which made little to no sense from an in-universe perspective as well as who the hell would be stupid enough to open the gates to a Tyranid invasion apart from Genestealers, the presence of whom does NOT constitute a mystery and has not been part of the lore on Sotha at any point.
Goulding can do some good stuff, and he's laid some good groundwork alongside Richard Williams, but I think the Scythes need somebody else to tell their story. Here's hoping that the Fall of Sotha gets its own telling one day. A better telling than Miral got.