Journalists/ reviewers aren't responsible for the "leaked" footage. If they do break their contract, then publishers may not work with them anymore (and in the grand scheme of things, reviewers with permission to play the game make up a very small group of people).
The short answer is that somehow, the game fell into the hands of somebody who knows how to dump the cartridge. It's possible that's because a store broke the street date. From there, once the game is on the internet, it's a matter of time before it ends up with dataminers and somebody who creates a torrent.
The footage you're seeing on Twitter, Reddit, etc. is from people who torrented the game, and are either playing it on an emulator or a Switch running homebrew.