Wasn't this because the photo was made "public" as in, shared through Google Photos?
Most people don't realize that it is 99% like posting it on Facebook.
It's not impossible that someone sees the URL, and if it's stored in an email the backups or local clients could store it plaintext, but I don't see how the URL is any more likely than the password to be sniffed during the connection itself
Its a random link someone would have to guess.