Long live cross-posting 
Anyways, as you probably figured out, 'idle' means doing nothing. Thus if you are working in another application you're not doing nothing, so Messenger will also detect that and wont set you as 'idle'.
Hence, the script is working as it should...
Same explaination goes for
[Help] Idle Time Display 1.0 {by Teario}
PS: you could always forcefully set your status to idle though...And since Messenger wasn't in control when you've set your status to idle, it wont set it back to 'away' or something else when you are doing something... I once made a script for that, search my posts 
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