Don't know how it is in UK but in the USA no one is allowed to send you stuff unsolicited that pushes some obligation onto you. Even an obligation to return a shipping error to them. You probably got a free Switch. And with their subsequent behaviour, a well deserved one.
US seems to just have a more broad term for goods delivered that you didn't order and refers to it as "unordered merchandise"[1]. You're pretty much free to keep it, it seems.
The UK is more specific for what is classified as "unsolicited" and what is considered goods delivered to someone else by mistake[2]. Companies have a right to get their product back if it's the latter. Since the author did order a switch, a duplicate delivery would very likely be considered a mistake. If the author never ordered a Switch (and never paid for it) in the first place, they'd definitely be able to keep it.
Regardless, if this is the way Nintendo asked for the second Switch back, they were completely out of line.
[1] https://www.consumer.ftc.gov/articles/0181-unordered-merchan...
[2] https://www.saga.co.uk/magazine/money/spending/consumer-righ...