If Browser checks that property... well documented... so, if that property is not set, then If Browser won't work. The only way to set that property is via either the Run Browser or Navigate operations.
While you are correct in TestComplete will operate against any browser open when opened manually, that operation has a very specific use in the feature set for testing across multiple browser types. It's purpose is so that, if you are running a test case in a browser loop or parameterized web test case, you can control different behaviors if the browser currently in use is a particular browser.
So, for your purposes, in order to develop against that, at least for the short term, add either Run Browser or Navigate before your If Browser call.