In order to answer the original poster's question, we need to decide what target panic is. The term has come to mean a mental condition outside of our conscious control that causes us to perform at less than our potential. If we perform at less than our potential because we are tired, bored, worried, or in pain, we don't call it target panic, because we can identify the cause and know that once the cause has been removed, the problem will go away.
Under that broad definition, I think we would have to agree that he has target panic, because he probably wouldn't be posting if he were just tired, bored, etc. If he didn't have target panic, there would be a gradual increase in group size as he increased his distance from the target up to the maximum range of the bow, not a sudden loss of accuracy at 20 yards. Now it is possible, of course, that if he shoots enough arrows at 20 yards, the problem might go away by itself, but it could also either get worse or come back at other times and places.
Personally, I really don't like that broad of a definition of target panic. I have experienced two different types of mental conditions outside of my conscious control that have caused me to shoot at less than my potential, and there may well be more.
The first type is where we turn the shot release process over to the subconscious mind, and the subconscious mind begins to look for shortcuts, as Joel mentions above. After a while, we are no longer able to reach full draw before the arrow is released. We find that we can't cure the problem just by deciding that we will not release the arrow until we come to full draw, because "deciding" is a conscious process, which has no control over the subconscious mind.
The second type has nothing to do with the subconscious mind: it is caused by doubt in the conscious mind. The symptoms are that we may feel a chill in our blood, like our brain is disconnected from our body, and like we have lost the strength in our arms and backs. We find it difficult to concentrate and focus on the target or anything else. We just want to get the shot over, hopefully without losing the arrow. It is similar to the stage fright someone might feel if they have an aversion to performing in front of a group.
What both of these conditions have in common is that the cause is difficult to identify and the problem can't be overcome by willpower. In fact trying harder just leads to frustration and makes the problem worse. I have overcome both of these, at least for the moment, but the methods I used were very different for each type of problem. Different enough that I believe it is confusing to call both of them by the same name. I would probably use "target panic" to describe the subconscious issues, because the term seems to be more frequently associated with those, in my reading, at least. I would probably call the second set of issues "doubt panic" but that's just something I came up with on the spur of the moment.
From the original poster's description, I would say he doesn't have any target panic, under my more limited definition, but may have a touch of doubt panic.