[Miky]

Obviously. But good luck implementing a human-comprehensible analog differential equation solver on one, without the help of a genetic algorithm, that doesn't depend (as Adrian Thompson's circuit did) on the temperature, the quirks of that specific board, and the effect of components which aren't even physically wired to it.

The difference isn't that FPGAs don't operate on analog voltages deep down (who said they don't?). The difference is in the set of tools and tolerances they give you, and in that sense FPGAs are only an analog coprocessor in the sense that a car can, technically, be used as a sailboat.