running on Unix, synergy plugin don't use the -n option to pass login
source code says
"// Unix commandline client does not support the "-n" option"

Running synergy 6.4.3410 this option is supported and a modified version of the plugin to use this -n option is working fine.

So it seems support for this option exists, at least on some versions of synergy client. The plugin should use it when supported - assuming it can detect it is supported