That is exactly what "modal" means, so, yes, you do want to make a modal dialog.
you said "modal dialog", only $input is modal, the dialog is far from being modal
The window that is created as a result of calling $input is called a dialog box
. Don't confuse the general term of these types of windows with the more specific "custom dialogs" that can be created with dialog tables in mIRC.
argv0 is saying that you want the $input window (which is a dialog box) to be modal.