Aaron Albert explains;
I get a lot of emails from publishers announcing their comic has been "sold out at Diamond", which basically means that the comics have been sold out at the distribution level. This doesn’t mean that the comic is sold out at the retail level, no there could be many comics still on the stand for you to grab multiple copies of. You can also take into account that a print run can be relatively small, even with the minimum orders recently put into place. So your only talking around 1600 comics (if you go buy some of the number thrown around lately). That’s all that’s needed for a publisher to throw out the "sold out at Diamond" phrase.
In my opinion, the thing I look for to see if a comic is really hot or not is if a publisher announces multiple printings. This shows that the demand is high and there needs to be more comics printed. For me, this is a much better indicator that a comic is doing well and is wanted.
I know the reason behind a publisher is to drum up some PR about their comic, it just feels like an empty praise. I wish there was another way for publisher to toot their horn, and for smaller publishers, maybe this isn’t empty praise, but rather an indicator that their comic is gaining an audience. Seeing it from the larger publishers though, sometimes leaves me with a slightly cynical taste.