Hi David -
You might have noticed that throughout Foundation 6's life cycle, there have been progressive enhancements made that are leading toward Foundation 7. This makes migration more gradual and easier for all of you. Since Foundation 3, the grid classes have largely remained unchanged. Flexbox offers more possibilities and it seems wrong to hamstring it with old classnames that restrict or confuse it's use.
The latest XY Grid released today with 6.4 does use new class names to better utilize the X-direction and Y-direction capabilities that Flexbox gives us. Thinking into the future, we purposely used the new class names because they work well with CSS Grid as well.
So going back to the first point, you are likely to see these classnames introduced today with 6.4 cary over into 7 to make a simpler migration.