While the current joycons are in need of a redesign, magnets alone wouldn't be very secure. That's asking for broken consoles and/or pinched fingers. If that turns out to be true, I imagine there is still more to the story.
Like, perhaps they could still have a sort of rail, but now they're held into position by strong magnets instead of a physical tab that requires manual activation to remove them (which were always a bit fiddly), just a smooth slide in. I know the article says "instead of rails" but it's easy for things to get lost in the game of telephone, especially when nobody was even allowed to look inside the box.
Whatever it is, there's gotta be some extra wrinkle to this attachment method (if true).