Can't speak to 256GB (I only use an 8GB, don't need my entire library in the car
), but it's possible that 32 or 64GB may be the largest fully-supported size and you could potentially be unable to utilize all 128GB of your card. So it's possible that it'll be able to read from a larger-than-supported card but unable to read everything from it.
But personally I'm a fan of understating what's supported just so people can push the limits reasonably successfully but can't hold you responsible if something acts up.
"I only get access to 96GB of my 512GB card!"
"Well, we did say only up to 32GB was supported, so if your card gets you 96GB that's pretty good!"