According to the thread on reddit, AT&T had been contacted and said that the site was in fact blocked:

http://www.reddit.com/r/reddit.com/comments/94pf2/att_is_now...

They of course don't say who called AT&T, so who knows.

The skepticism in your comment is apparent, but I thought I'd add:

The source is unnamed.

The source may very well be telling the truth. I've contacted customer service at AT&T U-Verse. I've never had fewer than three transfers and the person I end up with, while almost always being very polite and sounding as though they're smiling through the telephone ... generally has no idea what's going on.

The conversation could have gone like this:

Caller: It looks like 4chan is being blocked. I can't access it from my (DSL/U-Verse) connection. [... after a few minutes of sorting out what 4chan is ...]

Rep: I can't seem to access that web page, it says it's blocked.

[...click...comment...]

Just a guess :o)

Isn't the whole thing:

"Never attribute to malice that which can be adequately explained by stupidity...but don't rule out malice"

That's the Heinlein variation.

"Hanlon" is a corruption of "Heinlein", I believe.