If by "portable mode" you mean like [Vscode](https://code.visualstudio.com/docs/editor/portable) then no, Geany does not have a portable mode.
The `--config=` option sets where the user config data goes, its not related to a "portable mode".
I note that the link above says VS portable is not supported on installed versions of the the app, and Geany is only available as installed so it is the same. The default startup reads the users configuration, that is the condition that needs to work without options.
Of course nothing stops you "porting" the config directory between machines, but it has to be used with locally installed versions of Geany started with `--config=`, and be aware it is only forward compatible, ie we always try to make newer Geany versions read what older versions wrote, but can't guarantee older Geany versions will read what newer ones write.