If Multitasking Is Impossible, Why Are Some People So Good at It?

Multi core doesn't count since that's essentially two computers. And multiple ALUs in a CPU also don't count given that the brain has trillions of "logic units".

And in any case people multi task constantly - I can walk, talk, breath, enjoy scenery, look for obstacles, maintain cardiovascular pressure, digest food, maintain proper eye focus, and a really impressive number of other things (that are all brain controlled - not independent) in a true simultaneous fashion.

And if you mean multi task higher level thought - people do that too. How many times do you suddenly find the solution to a problem you are working on while doing something else entirely?

The only thing we don't multi task is attention. And that's by definition not by reality. Something that can multi task attention is considered to be two somethings rather than a multitasker.