Non-users can't access forum link?

I recently linked my card game forum discussion to a couple of friends, but they were unable to access the page. I am thinking that it may have something to do with the age of the forum post, or maybe it has to do with that little lock icon that is present next to the library category. Does anyone know what could be happening?

For reference, here are the links I have used:


I believe that’s what the lock next to library category means.

That’s too bad, oh well then.