I was also under that impression that the 0/0 ratings were "unlimited" as well, but I wanted to look further into it.
I tested it using all of the same units and same situation:
With ranges numbers actually entered, the a/c immediately took off and intercepted the bogey as soon as it was observed, even if it was unidentified. In other words, it seemed to work great.
Without a range, it allowed the a/c to continue to close on the target a/c as long as it was still unidentified. After a couple of minutes without making any attempt to intercept anything, I then manually changed the target from "unidentified" to "hostile", and the a/c immediately launched to intercept it.
My conclusion is that, while both methods do work (and there are plenty of scenarios with no ranges listed), the scenarios with actual ranges perform much better.