A pressure vessel does a bit more than a tank. While a tank may house liquids/gasses, it typically does not regulate or maintain a certain amount of pressure that is different from the natural, ambient pressure.
Anyways - perhaps it was my misreading or an edit after-the-fact by the parent (can posts even be edited on HN after responded to? I haven't tested or taken particular notice of it), they seemed to be confused that the statement switched from talking about an oxygen tank and then a helium tank (as if both were the same tank) and were confused about which gas the tank contained, when in fact there are 2 containers and not 1 with the gas being mixed up. I was simply clarifying that. There is a helium pressure vessel and an oxygen tank - not an oxygen tank that is later called a helium tank.
I don't remember seeing the question appended at the end when I responded: