I was looking at the decompiled source for DamageWorker.ExplosionCellsToHit and I noticed that in the second loop, it only adds cells to adjWallCells if adjWallCells already contains that cell. Since adjWallCells starts out empty, this means that the second loop will never do anything. Not sure if this has any ingame effects, but it seems like it must be a bug - I suspect the intent was to check that adjWallCells does *not* already contain the cell.
Great find, it's a bug indeed. Thanks! We'll fix it.