To me, the definition of a "general purpose" item is that it is suitable for a variety of uses, but not ideal for any of them. By contrast, a "special purpose" item is very good at one task, but performs poorly outside of that arena. For example, a "general purpose glue" is good to have around, since it will stick many things together adequately. But if you are joining aluminum to aluminum and want a permanent bond, you'd want to use a special purpose epoxy that is designed for this application.
As a parallel, consider the statement "the minivan is not a general purpose vehicle, as the existence of race cars (including F1 racers) indicates that all attempts thus far at building a general-purpose vehicle have failed." I feel this claim would be nonsense, because (obviously?) a vehicle that has been specifically designed for a particular purpose will perform better at this purpose than a vehicle designed with many other higher priorities.
I also tried variations of "a vice-grips is not a general purpose tool because it makes a lousy hammer", "Webster's is not a general purpose dictionary because it lacks full coverage of Latin medical terminology", and "a pencil is not a general purpose writing instrument because it will not scribe metal". But these all seemed so ridiculous that no one would claim they made any sense. So I removed them.
How did you find it insightful? Maybe "general purpose" is used differently between American and British English?