To be clear, open matte doesn't mean 4x3. It also doesn't mean cropped (as the ebay picture might suggest).

It means that mattes have been removed to show more image. In the case of TGMD, it was theatrically 1.85:1. Opening the matte SHOULD give 1.66:1 if animated in those dimensions (it was shot on 35mm which is 1.66:1 but earlier animations were frequently drawn at 1.33:1).

But, as the 2010 release shows, 1.77:1 is actually open matte compared to the 2002 1.66:1 release. That doesn't make sense either. Based on that there should be more height to the image somewhere as well.

What Disney has actually done is pretty much a mystery. The OAR is just unknown.