There's multitasking, and there's multitasking.
The kind of multitasking humans don't do well (and it's not true they "cannot" -- just that they do not do it well) is the equivalent of parallel processing in a multicore computer. The kind of multitasking consultants need to be able to do is more like pre-emptive multitasking, which works even with a single-core single-CPU machine, where simple tasks are lined up in a priority queue so that needed progress can be made on multiple complex tasks over a given period of time, rather than having to complete all of the simple sub-tasks that make up one complex task before tackling any of the simple sub-tasks that make up another compex task.
If people couldn't do that, they wouldn't be able to go for a walk, because they need to be able to keep track of where they are going, how multiple people around them are moving, and the conditions of their bodies themselves, sufficiently well to avoid getting lost, falling in a ditch, colliding with other people (or cars), or arrested for wandering into a restricted area (for some examples of possible problems). We keep track of all these different things going on while walking around by switching attention between different concerns in a constant series of context shifts, prioritized by immediacy, situational stability, and danger level.
So, yeah -- people can multitask just fine, or a particular definition of "multitask". It's called "time management" when you want to avoid someone nitpicking based on a different definition of "multitask" than you are using.
