Superglue itself is not toxic to dogs or humans and superglue is not poisonous.
However it's still not safe for dogs to ingest superglue or even humans because if either humans or dogs ingest superglue it can cause health issues and possible death if not treated.
If a dog happens to eat a tube of superglue then they may experience health issues such as vomiting, coughing, stomach pain, diarrhea and even possibly glue your dogs gums shut.
So if your dog ingests a tube of superglue you will probably need to take the dog to the vet especially if the dog gets their mouth glued closed.
The vet would have a way of getting the superglue dissolved naturally so that your dogs mouth becomes unstuck.
But the best thing to do is to keep superglue away from dogs and even small children who put things in their mouth because if a toddler or young child puts superglue in their mouth it can glue their mouth shut as well and then they would need to go to the emergency room to get the superglue dissolved if they happened to glue their mouth shut.