I think that was for a simpler problem, that shared urls were not unique enough, and could be guessed or brute forced to get random peoples shared items. The articles published by google seemed to warn that there was an expectation gap on the amount of privacy people should have expected to have on them.

That might be a "we need to change how we're generating URLs because we want to refactor stuff / not support the old format because it was too guessable or something" thing.