How would one output `Diagnostics` inside a build command?

It seems like they're meant to be used only from the plugin itself and not the build command tool? If so, it severely limits their usefulness since most of the time the build command tool outputs the most important info. Is this the case?

Not sure what you mean but you can find the output of the build tool inside xcodes tab build output. Look for apply plugin ... or ran custom plugin. Then you have to click on that line to expand the logs the tool has output. Is this what you mean?