Actually in the US if Amazon accidentally sent you a TV that you never ordered you would own it. As per the FTC it's considered a gift and you legally don't have to return it. There is info about it on the FTC's website:
Billed for products you never ordered, or for products you ordered but never got? Federal laws protect you. Here's what to do.
consumer.ftc.gov
If this was legally purchased by the Youtuber from a distributor who legally purchased the product from WotC and the distributor sent him the wrong product he is legally entitled to keep it. It doesn't matter that the set was not supposed to release for a few weeks. That's an issue between the distributor and Wizards of the Coast. This Youtuber didn't sign anything with WotC saying he wouldn't purchase the product early.
If this Youtuber received this by having a distributor accidentally send him the wrong set and he is not under any sort of NDA with Wizards (which I don't believe he was based on what I've read and the size of his channel) he owns the product and is fully within his rights to open up the boxes on Youtube.