A quick check suggest that this behaviour has always been there and is not a recent change. Interestingly other dialogs also appear to be modal including About. Also FGL_WINMESSAGE, which is known that when this uses the System API the resultant window is also Modal and blocks other Genero applications running on same GDC instance. (see important tip here
http://4js.com/online_documentation/fjs-fgl-manual-html/#fgl-topics/c_fgl_utility_functions_FGL_WINMESSAGE.html) One window that wasn't modal was the Console Show window which has to be non modal in order for it to be useful.
I'd wonder if being modal means we don't have to worry about the case of the same shortcut being edited in two different windows, or the user clicking Delete on a shortcut that is being edited in another window, or the user clicking Start in a shortcut that is being edited etc