The reasoning is that once the proposal here is applied to Showcase of Languages, answers to https://codegolf.stackexchange.com/q/100928/26997 would be valid answers there, which allegedly goes against our usual duplicate rule of:
Can answers from one question be copied over to the other with little or no modification and still be competitive? [If yes, then it's a duplicate.]
Now this may be true, assuming the proposal] about removing the one-char-per-upvote rule in the old showcase is implemented (the "Make it a non-challenge question" thing). But the old showcase has been locked for nearly a week and none of the changes (proposed 4 days ago) have been implemented.
Dennis even says (in response to Sp3000's comment):
Fair enough. Once we settle on making an exception, we should probably have a separate poll to decide what the conditions are. As I've mentioned before, I do think this is the most important detail though. [The removal of the one-char-per-upvote rule.]
So it seems likely a new meta post finalizing exactly what to do with the old showcase will be made before it is unlocked or changed. This process could take a week or two to decide and more time to implement, and the results cannot be definitely predicted.
Given the present state of the old showcase, I'd argue that the new one is by no means a duplicate. It would be a very contrived answer indeed to the new showcase that follows the restrictive rules about character counts and votes present in the old showcase. The old showcase is still locked in any case, so people who see the new showcase and want to answer cannot do so on the old showcase.
(I realize there are other reasons and arguments to be made about why or why not the new showcase should be closed. (In fact it was closed as too broad, but last I saw it had 4/5 reopen votes.) But the new showcase was marked as a duplicate specifically because answers could supposedly be copied to the updated old showcase, so that's what this meta post is in reference to. Please focus on that facet of the discussion.)
It seems likely but not certain the proposals will be implemented.
If they are, at least 3 people agree (2 comment upvotes currently) that the new showcase would then be a duplicate of the old one.
Either way, it will likely take a week or two until the issue is finalized.
In the meantime, no one can answer either showcase (the old one is still locked).
Should the new showcase remain closed as a duplicate of the old showcase despite that it is not yet a clear duplicate?
(Note: I'm using the terms "new" and "old" showcase for convenience because they are the only showcases we have so far. I do not consider one a replacement for the other, nor that they will be the only showcases ever.)