It is not border-humping skytrain. The skytrain went out of the north border, disappeared from the map (could only hear the sound) and came back at the harbor. It is the typical out of bound skytrain exploit (cheating). The ZGL fleet learned this from another fleet that is no longer active. Unfortunately, with no action from GM, this exploit "disease" have already spread long ago.
When we play this game, we want to fight battles (win or loss). For ZGL, they have a different objective. The conclusion is that there is no point to play HD against them.