Well, which representation have they made? What do reasonable or ordinary members of the class of purchasers think it means? These are not problems for this thread, so let's put the ACCC to one side.
I'm sure, really, that we must be referring to the species of cotton plant here. The question is, do you prefer the shirts because they're made from cotton of that species? I imagine not, because the other species of cotton plants have (apparently) been cultivated to have similar properties, i.e. they also have a "long staple" allowing thinner and stronger threads. For all practical purposes it means nothing, even though there is a theoretical distinction, so you're probably just partial to a 2x100 shirt.