Ironman seem to tell lots of stories to try and get sales....
A few facts:
As already stated by Nismo, ARB tanks aren't in fact "ARB" tanks at all - they are "Longranger" tanks and you can buy them from lots of different places. Made right here in Australia. ARB don't make them themselves, they sell someone elses product. Similar to the situation with IPF lights - ARB don't make them, they just sell them.
The "ARB tank" for the D40 is an "add on" tank - the Ironman tank is a replacement tank. There are two schools of thought on which is preferable, however I will let you draw your own conclusions on whether the come from the same factory seeing as though they are obviously different designs...
Makes me wonder - why do they feel they need to tell these porky pies? Are their products flawed? Do they feel inferior to the other players in the market? What is inherently wrong that they are lying to their customers in order to try and make a sale?