It could be a Steam Curator key in which case it is effectively like a retail key for gameplay purposes and permanency. I'd go a step further and state that it is almost certainly a Steam Curator key as the beta for that game has long passed. So you should be able to play that game indefinitely with that key. Developers cannot revoke Steam Curator keys from what I understand. Obviously such a key has a description that is different and it is listed differently in your overview of game licenses. Some want nothing to do with Steam Curator and others do not mind. Obviously for developers it can be less beneficial if Steam Curator keys are used in this sense. It depends though....they hope to increase visibility for their games by means of hopefully positive game reviews (and possibly tips / hints which help improve the game further) which hopefully in turn lead to better sales.
There is no way to delete a PM via Steam. Generally it is best if people specify exactly what they are offering to the other party, so that no one is unhappy in hindsight. Obviously that does not always happen. It is best to talk to the individual you wanted to trade with first to see if you can work it out in a way that works for both of you if it is a major issue, assuming it is a Steam Curator key.