The article literally talks about how one of the server implementations does exactly that:
> Does this affect Prosody?
> Not directly. Let’s Encrypt is not the first CA to issue server-only certificates. Many years ago, we incorporated changes into Prosody which allow server-only certificates to be used for server-to-server connections, regardless of which server started the connection. [...]
> Does this affect Prosody?
> Not directly. Let’s Encrypt is not the first CA to issue server-only certificates. Many years ago, we incorporated changes into Prosody which allow server-only certificates to be used for server-to-server connections, regardless of which server started the connection. [...]