No, I think it's more a case of the lawyers will always find ways of failing to catch everything that they should catch.

When the premium cabin rate of air passenger duty was introduced (I can't remember if it was there right from the inception of the idea), the lower rate was payable by passengers in the lowest service class/cabin on the aircraft; everyone else paid the premium rate. If there was only one such class/cabin on the aircraft, everyone paid the lower rate.

I'll bet that the departmental lawyers who drafted the legislation either didn't appreciate that a single premium-class only aircraft like Concorde would fall into that definition, or alternatively decided that it wasn't worth it (for 14 flights a week, carrying a maximum of 100 passengers each) to try to come up with some more convoluted definition that would compare the service in the cabin on the single-class aircraft with the service in the cabins on other aircraft operated by the same airline.

As I understand it, you should still be able to use the benefit of this definition by flying on the single premium-class only aircraft being operated from Stansted at the moment. And if you fly MaxJet, you don't even have to be particularly rich to take advantage of it.