Not making an argument, but thought I could be of some help. I think
this is the most prominent place where it was officially stated the Duck cannot be used at Level 9. It could use a follow-up post to ensure it doesn't trip up others. It was also brought up
here, although not as strongly. Those threads are the reason why I didn't think the card could be used at Level 9 up until today, in any case.
And I'm glad for the new ruling as it no longer means figuring out whether you have enough cards to be able to handle all the various discards before figuring out whether you can play the Duck either.