They're being purposely coy though on what this actually means. Backwards compatibility with digital/e-games, or backwards compatible with the physical carts?
There isn't a technical reason to change the cartridge format. I don't see why they wouldn't just use the same carts if backwards compatibility is the goal.
There are now cart dumpers that can copy and store multiple Switch games on an SD card. If the same form factor is used its likely these will still work for original Switch games.
I would be unsurprised if the cartridge has the same form factor but has internal differences for Switch2-only games. If they want to try to lock Switch2 cartridges down more, there's plenty of ways to do that while maintaining a similar enough form factor for Switch1 compatibility.