Nintendo's plans was to launch Advance Wars 1+2: Re-Boot Camp on April 8, but as the conflict in Ukraine started, Nintendo decided to delay it as a result of the war theme. Some people had pre-loaded the game already though, and it seems like a few of them actually got to play the game this weekend as it unlocked as planned.
A reasonable guess is that people who didn't have their Switch units connected to the internet, simply didn't get the update with the new release date and thus got to play it. A video has been shared on social media show what it looks like.
While we still don't have a proper release date for the game, we assume it won't be too far off, as it is already finished. And if you really care about spoilers in a game like Advance Wars 1+2: Re-Boot Camp (it's a remake of titles that are not too story heavy to begin with), it could be an idea to tread lightly online.