I don't know why that doesn't show up on the extend site. How WordPress extend site works is kind of a mystery to me. :-/
As far as the username/password, if you are defining those correctly I don't think you should see the rate limit stuff anymore. Does it give you the warning every time, or just the first time and then a blank gist after that?
Could you try moving the defines higher up in the file? (maybe to the top?) See if that makes a difference? I don't think it should but it is worth a shot.
If that doesn't work, could you try changing to the wrong username and password and tell me if that changes anything? I want to try and figure out whether it is related to the _USERNAME and _PASSWORD fields in any way. If you put the wrong username and password in you should (hopefully) get a different error.
Can you verify that the version of the plugin that is actually installed into your WordPress installation has the _USERNAME and _PASSWORD code?