This is a non-issue. The URLs are way to unique to guess (you'd have an easier time guessing an email/pass/2FA). And ones ability to access the URL at all is the same as their ability to access the bytes of the image. Once accessed, they could capture and share either.
Agreed, I was looking for something more substantial than this, too. I was thinking it was some clever unpatched way to scrape semi public Google photos links. Turns out it's just the sharing feature working as expected.
This would be an issue if it were mutable data.