There is just no excuse for not even salting or SOMETHING to keep the secrets out of plaintext. The reason you don't store in plaintext is because it can lead to even incidental collection. Say you have some software, perhaps spyware, perhaps it's made by a major corporation so doesn't get called that and it crawls around and happens to upload a copy of a full or portion of the file containing this info, now it's been uploaded and compromised potentially not even by a malicious actor successfully gaining access to a machine but by poor practices.

No it can't stop a sophisticated malware specifically targeting Signal to steal credentials and gain access but it does mean casual malware that hasn't taken the time out to write a module to do that is out of luck and increases the burden on attackers. No it won't stop the NSA but it's still something that it stops someone's 17 year old niece who knows a little bit about computers but is no malware author from gaining access to your signal messages and account because she could watch a youtube video and follow along with simple tools.

The claims Signal is an op or the runner is under a national security letter order to compromise it look more and more plausible in light of weird bad basic practices like this and their general hostility. I'll still use it and it's far from the worst looking thing out there but there's something unshakably weird about the lead dev, their behavior and practices that can't be written off as being merely a bit quirky.