It seems superfluous for the default format to display the channel name, when allow_channels is set to false, because I think that means there will only be one (Global) channel?

Yeah, I suppose this is true, however the channel manager still handles targets (%t variable) for all messages anyway, and the format string doesn't adjust anything based on config values (target is target, regardless)

You should be able to manually remove the target from the format string without any impact on the config.