If someone is "really good" at texting while driving (let's say top 1%), it doesn't mean that he is anymore than average at a different multitasking task, like solving math problems while remembering a string of letters!
For a computer any bunch of tasks can be considered equivalent, they are just calculations (yeah, even in this case we have to ignore a lot to say this but bare with me), but the human brain uses different physical subsystems to handle different tasks. So any multitasking performance depends specifically on the tasks being multitasked. Yes, there are things like general attention and interpresonal differences in it, but for the wetware in our skulls, the idea of "general multitasking" and being better or worse at it is bullshit imho.