That is fair if the code is 100% owned by signal.
But, please follow me on this: since nobody is supposed to run #signal servers but themselves, you would agree that the AGPL label is used a mere marketing billboard. As @IngaLovinde correctly pointed out in this thread, it already did not guarantee that what you see is what you get as service.
Today all the doubts about the actual software running on the server side are gone. You can be sure that they won't share their sources, so you cannot know what they are running.