Yeah, that's what I used to think too... but then someday I've tested them outside of picwins, just for the sake of a trial, and they worked.
I believe they've always worked outside, but I don't know for sure. I know it's been a long time since I've tested, though... mIRC's version was 5.4 or something.
Edit: looking through
versions.txt, I've found that it was exactly mIRC 5.4 that introduced this: "53.Can now use $mouse outside of picture window events [...]". So yeah, it's not always been like this.
