Hi Russell,
In my view it's not what the ship is doing that determines it's classification, but what it's designed to do. This is clearly an inland tanker, the fact that it is bunkering another vessel is secondary.
To illustrate further, there are many seagoing chemical and product tankers in service undertaking bunkering ops. Some switch between bunkering ops and general short-sea trading in the fuel oil sector when demand swings one way or the other. Theoretically, they can also go back to trading in chems or CPPs should the owners wish. Should these all be considered for the port bunkering category? I don't think so.
On the other hand, there are other ships permanently engaged in bunkering either having been cascaded down from the chemical/CPP sector for being overage (they will never go back to trade), or newer specialist vessels which have been designed and built as bunkering tankers.
Brgds
Phil