The “backwards” grip of Zatoichi and Ahsoka is purely movie magic. It’s completely impractical and nigh useless in real combat. There might be a few random techniques that briefly use a similar grip for very specific movements before switching back to a more normal grip, but I’m not aware of ANY martial art that uses that as a standard grip for the basis of the fighting style.
And for what it’s worth, it absolutely doesn’t improve your grip strength. One of the first things you learn in Kendo is how to grip the sword. The strength is in your thumb, pinky, and ring finger. It’s hard to explain without demonstrating it in person, but it’s absolutely true. And it’s true with all hand held weapons from katana to spears to knives to rapiers to fencing foils.
Heck, it’s the reason modern fencing grips look like this:
Instead of this: