Yeah, Getty is selling photos that are public domain. Photos in the public domain are already free to use, though some people might pay to have peace of mind that they didn't take a similar, not-public domain work by mistake or something.
From their license
> Warranty of Non-Infringement. For all licensed content (excluding content marked “access only”), Getty Images warrants that your use of such content in accordance with this agreement and in the form delivered by Getty Images (that is, excluding any modifications, overlays or re-focusing done by you) will not infringe on any copyrights or moral rights of the content owner/creator.
https://www.gettyimages.com/eula
Note, I have many photos CC-BY on Flickr. One company wanted to use an image I took in the Louvre and wanted me to sign a different license. I refused as their license required I indemnify them and I had no idea what the legality of that image is given it was taken inside the Louvre