They are overstating the case a little bit, but this is still a big step. These images have actually been available since 2011; back then, until now, it was under a license very similar to the old Creative Commons attribution-noncommercial. But it was not so easily understood as a CC license.

What they have done recently is assert that images of artworks that are public domain are also in the public domain. Other museums and institutions have done this, but for the Met to do it is a big deal.