trough, an animation like that wouldn't work without uneven pixels, so he has to create a resolution mix. I agree that creates some less than perfect artifacts, but the end result is probably still better than trying to do the subtle animation in pixel art, probably with subpixels. Also, the way he does it right now, he can do the animation in-engine with deformation rigging, which he couldn't with the other method.
Onepixel: when working like this, you need to work hard on avoiding the worst artifacts. For example, the grass on the right tree was mentioned, because it creates a very sloppy effect when resolutions mix like that. Since the grass isn't animated, that should be easy to avoid.