Yes, the pictures are still in the "RICOH THETA" folder. I can access them using Google Photos or File Manager. For good measure, I tried moving all the old pictures out of that folder, and then moving them back into the same folder. There's still nothing shown on the Theta S app.
PS: I also found out that if you were to move a picture out of the "RICOH THETA" folder, it will also disappear from "Device Images" in the app, even if you move it back into the folder later.