This is simply not correct, the page you link is talking about problems of email as a network of different clients and servers. With ArcaneChat and arcanechat.me server there is no metadata leak, the article talks about leaking subject which is simply not leaked in ArcaneChat since it is moved to the encrypted part as many other headers, the To and From headers are needed by the server to know to whom send the message, this is the same in virtually all other messaging platforms, like XMPP, Matrix, WhatsApp, etc. So why is it listed as a flaw of email?
Here you can see what someone can see in a message sent with chatmail servers, tell me exactly what metadata you got from this message as the server operator:
That kind of "no no you can't use email in a secure way" is a so outdated urban legend
it seems Pleroma is really bad, you have to scroll down until you find the actual comment in that thread that is marked with a different color, it is showing you the top of the thread, anyway, here is the image: