1. Signal planned this as a PR stunt, anticipating that Facebook would reject the ads.
2. After setting up their ad account, the account or payment method was flagged for additional verification, triggering the dialog they showed in the screenshot in their blog post.
3. That dialog has a big red padlock that says "Ad Account Disabled". That's perfect for Signal's needs, so they stop here and don't even bother getting the actual ads reviewed and rejected.
Signal's post is careful not to say that the ads were rejected, only that their account was banned.
In response to the pushback, on Twitter, they posted another screenshot showing that the account was restricted, but still no evidence that they actually attempted to run the ads.