What I think, this game has so many new content and kind of exceeds what other similar type of games hold, that there's bound to be bugs.
I'm only a 3D modeler but when I come across a bug (I'm used to calling them bugs than glitches), it just seems that it was overlooked or it was so minor. QA testers are trying to break the game constantly before it gets released. Sometimes game studios will pay people to playtest their product (don't know if Nintendo does this)
I'm thinking QA testers are looking for big bugs like oops the character fell through the world. Oh I got stuck on that corner.
So when you have these dupers, they're breaking the game in a different way than the character collider getting stuck, etc.
The dupe glitch, I was trying to figure it out. The gist of it, is it basically breaks the item collision while in the editor (placement editor?) I can't even imagine what that code looks like for the editor. There's bound to bugs within that. No doubt.
The game design pipeline is not an easy task.

happens. It's terrible when some breaks and you're just like wtf, when it worked perfectly 5 mins ago.
And so these dupers are constantly on the hunt for cheatsy ways. This is what they do. This is their lives in some way. If the bug gets patched, they're on the move for something else. They are tunnel visioned compared to QA testers who are bug generalists.
That's my basic defense.
I'm sure Nintendo is aware that these people exist. Maybe they didn't expect them to move so fast, like who knows.