I don't know, and I suspect that the task of tracking down definitively and conclusively exactly what the conditions are when I can and cannot read things would be lengthy and relatively unrewarding.
I have better things to do than play "hunt the foible" on Google.
In my experience, the conditions are pretty much as described.
If Google knows you have an account because of tracking devices, they require you to log in because you are more salable.
If you're anonymous, you're more salable if they just show you the content.
In so far as I recall, originally G+ wouldn't show content without javascript turned on - i.e. the price of content was to allow Google to track you more closely.
I did try the link, it did demand that I login. I did not login, and it did not show me the content.
I resent such accusations, and suggest you make sure you are in full possession of facts before you do so.