That’s fair, though personally I’m kindof glad they did. “Signal is a secure messaging app” is a lot easier to explain to non-tech-savvy people than “Signal is a secure messaging app, as long as you are messaging someone who is using Signal too. It can also send regular texts but they can’t be encrypted.” Leaving that nuance out would have left people texting with a false assumption of security, but I lost several people explaining it because it “sounds complicated”.
Largely, yeah, we just don’t know. In research terms, it’s still quite early, so anything definitive is likely years away. However, research is starting to indicate it may be neurological - a product of damage to the brain and nervous system, as this article discusses.