The tweet doesn't say that these photos came from "Photos", just an "unused account". All other evidence seems to suggest that these are photos from another service called Panoramio which Google acquired at some point. Not saying it's right or wrong by the way (perhaps something in Panoramio's TOS that all photos were public), but based on this news I'd assume that my beach photos in Google Photos are safe from prying eyes - for now.
Strange... Among all of my google accounts I have checked and NOTHING was automagically shared. My pictures must suck or I did NOT do:<p>Share a photo
On your computer, open Google Maps.
In the top left, click Menu Menu and then Your contributions.
Choose Photos.
Find the photo you’d like to share.
In the top right, click More More and then Share.
Same for me, but they took my 2009 photos from an old OSM-based website with panorama in its name that I used back then.<p>Can't remember the exact name of the website, though.