The authentication methods are tried until one succeeds or none is left.
It’s the same for Julias download as it is done by libssh2.
Your snippet is not the right part for your issue.
You need to look for host keys, kex and ciphers. Those tell you, which your client (or filezille) is able to choose for your server. Your client and the server are negotiating them.
If it is something, which libssh2 doesn’t support, it would be a hint on whats going on.