The problem with it being the "sharks fin" is that it doesn't look any different than what we're used to seeing, so we assume it's the thing we already know. It isn't dwarfing anything in that shot letting us know it's something bigger and badder than the SDs we're familiar with. It isn't until the shadow is cast over the SDs that the audience thinks "Whoa! What's this gonna be?"

Also, as Niflamol pointed out already, it seems to be going the wrong direction in the "shark fin" shot as opposed to it's full reveal. The tower is going from the right to the left of the screen, but the SSD is moving from the left to the right. You could argue that we're viewing the tower from the other side in that shot, but that isn't how the language of cinema works.