You didn't read the entire article, did you?
The bulletin warned of Russian professional hacking groups employing phishing scams to gain access to encrypted conversations, bypassing the end-to-end encryption the application uses.
Phishing scams are very common and everyone...whether they are in the government or not...should be aware and not become a victim of the scam. That is what the memo is about.
And Signal points out that their app is not any more susceptible to phishing than any other app.
Signal responded to the bulletin in a social media post Tuesday, saying the NSA's "memo used the term 'vulnerability' in relation to Signal-but it had nothing to do with Signal's core tech. It was warning against phishing scams targeting Signal users."
"Phishing isn't new, and it's not a flaw in our encryption or any of Signal's underlying technology," the company said. "Phishing attacks are a constant threat for popular apps and websites."