Common misunderstanding, but that law does not apply to this scenario. That law is designed to stop scammers from sending you a box of toner cartridges then billing you for it hoping you don't notice you never ordered it to begin with (just an example). It is aimed at things sent to you unsolicited.https://www.consumer.ftc.gov/articles/0181-unordered-merchandise
If you didn’t order it but you received it, you can keep it and you can inform seller at your discretion.
That is not what happened here. Nobody sent @Mac'nCheese anything. The package was apparently sent from Amazon to the real owner and was somehow accidentally delivered to Mac'nCheese (if I am understanding correctly). In that scenario, keeping the package would be theft. Not saying anybody would bother to prosecute, but it would still be theft.