This isn't what your screenshot is referring to, but you can introduce a vulnerability if you use a port above 1024. Ports 1-1024 can only be bound by root. If you run ssh on port 2222 for example, anyone who is able to crash that process can then start their own ssh server on port 2222.
Say you have a website with a vulnerability that allows them to take control of your Apache/Nginx web user (all too common), and your ssh server has a vulnerability that can crash it (less common, but also much less severe). The web user can crash the ssh server and start their own, and because it's running on a port above 1024 nothing will prevent it. As soon as you send your password to that ssh server (either to log in, or using sudo to gain root privileges), the attacker can gain whatever privileges you have as well.
I wouldn't call it a high severity vulnerability, but if an attacker can gain access to a non-root user on your server, and your server is expected to run trusted services on ports above 1024, it can be an unnecessary weakness.