For the sake of clarity, it probably makes sense to call a salmon a salmon, and a trout a trout. "Salmon trout" is just like calling a certain African bird of prey a "harrier hawk": it can't be both, that just doesn't make any sense.
Anyway, restaurants and fish mongers are well known for serving/selling inferior fish under the guise of a fancier species. I'm of course in no way suggesting that's the case here, but it is apparently quite common. When it comes to fish, consumers are easily fooled.
Johan