Pegasus tries to get root on your phone. If it succeeds, it could theoretically read message content or decryption keys right out of RAM and Signal doesn't have many options to defend itself.

If it doesn't, it tries to get additional permissions by other means, including asking the user for them. If it gets permission to read notifications and Signal is set to show message content in notifications, then it can exfiltrate your Signal messages. Your messages might be safe otherwise.