I don't believe this to be axiomatically true. I don't believe it's an intrinsic ability, but I believe it's very possible for humans to task-switch at a level that appears like multitasking (and when you think about it, the only multitasking modern computers do is the threading that multiple CPU cores enable, when it comes to a single core there is only one task running at a time, ever).

In certain situations, I've found my brain able to do almost fractal searches through a target space. Or at least to simultaneously search for the answer to more than one question (take the game Hangman and its variants, when looking at a partial word my brain is certainly not searching for a single letter at a time).