Multitasking (at least for some definition of the word) is possible and I do it; I have done it for many years. Maybe if I could single-task I would be more efficient; I will never know because I can't. And yet I am efficient and 'get shit done'. Maybe i'm doing simple stuff; I usually am writing coding, chatting on skype/wechat/slack, browsing the web and having some movie or tv show or youtube tech event video on (I pick up enough to enjoy it and learn from it in the latter case). It works fine and has worked for 20+ years.

The exception is when someone feeds me dense enough information that I cannot process together with other things but I cannot focus on it very long without needing to reprocess it which is why that rarely happens and this kind of information is usually provided (and requested) on paper.

So yes, I agree with you; I have never been diagnosed with anything but I always have been capable of getting work done while others are sitting through slow talking people in meetings (usually the case). It actually helps me not just completely shutting the presenter out and hear nothing at all.