They're being purposely coy though on what this actually means. Backwards compatibility with digital/e-games, or backwards compatible with the physical carts?
I'd be shocked if it doesn't support physical carts, given Nintendo's history with backwards compatibility. And given the rough equivalence of digital games and carts on the Switch, I'm hoping that means digital purchases transfer too - but that would be a first for Nintendo, I think. Fingers crossed!
From what I understand, people are much more into physical media in Japan. Nintendo also actually finishes their games and gets a working build ready before release so the carts actually have a game on them that don't require a patch, which is unfortunately not standard across the industry.