I see what you mean, so let me try to explain:
You don't have an ID for the account (like a username, random number or even public key) but instead an address for your chat with someone (this is the queue where your chatpartner sends their messages to). So every address is unique only per chat, no unique ID per account or so.
If you still have questions about the SimpleX system, feel free to ask and I will give you useful links.
Desktop app does not work without the phone? That is just plain untrue. You mean on the same account? Then yeah, it is weird to only have this link one-sided - but it is cumbersome anyway, so much more convenient to use two accounts.
Finally! Their deb was broken even on Ubuntu, and Appimages are no real option.
But the size is insane, Electron is really an issue.
Also, the app just works if your phone is in the same LAN, and requires an open port which is also randomized, so secure firewall configs are very problematic, as you need to open the port manually every time.
You can use your own self-hosted servers with this too.
If you want.
Self-hosting can create its own additional privacy and security issues... unless you totally trust not only the place where you put the server, but also yourself to be a security expert