While the GGP idea was just misinformed and basically outright dumb, it doesn't mean that just because a politician chooses to use a tool or a person a a given time then they will protect that person or tool in the future. The world is full of examples in the contrary.
Bottom line, just because Hilary's campaign is using Signal, it doesn't mean that in the future her administration won't gag them or make legislation available in order to use them so spy on their own citizens.
Moxie's link shows her campaign is in fact relying on the service.