Especially since Signal positions themselves as a "privacy and security by default" messenger, one could expect this to not be an oversight.
iMessage does encrypt the keyfiles on MacOS. That's maybe comparing apples and oranges, but it shows that much more is possible and in my opinion expectable of the Signal foundation.
So basically you're safe on Android or iOS because of the much better security model, but on any desktop OS you'd have to make sure that there'll never be a malicious app or script running in the background. That's something probably 99% of Signal users are not capable of. They got (talked) into using Signal believing that it's not necessary.
I'm mostly disappointed by the smug answer, that they don't even recognize this being an issue. Yes WhatsApp is no safer, but security and privacy is why people switched to Signal, the bar is higher than that.