You're absolutely right. I'm even trying to be careful about the use of the term "firewall" to avoid giving the wrong impression.

I made the conscious choice to only block applications that play nice. It should cover most of the use cases. If there was a malicious app on your system, it would probably be impossible to even select it using the current UI.

I have no idea how to make the arms race against malware nice, lightweight or unobtrusive. With this software, I did not even attempt to.