There is an argument in Erich's favor, all this shooting happens at the end of the opponents movement phase. Which you could be forgiven for thinking that meant it was simultaneous. But I don't believe it, it is sequential, much like Deep Strike all happens at the beginning of the phase, yet one unit can mishap if lands on a unit that landed "earlier".
Still though, I believe a signal interceptor gun can only shoot at one target, whether that's the transport or the transported. Unlike Coteaz. Coteaz gets to do everything, all the time.