My eldest son is a whiz- he's a couple years ahead and should get through AP Calculus and Stats by the time he gets through HS.
On the other hand we adopted 5 girls from foster care and it is a STRUGGLE. I don't know how much of it is organic (all of them were exposed to drugs/alcohol in utero) and how much of it is early formative, but they all have incredible difficulty making the most basic inference or deduction or story problem. I'm really concerned for them because I forsee them potentially running into the roadblocks referenced by the article summary. But there are in fact SOME excuses for saying "I can't do maths." Some people may never be able to master the basics no matter how hard they try.
Not to say we are in any manner giving up. They get extra tutoring at school and spend hours doing homework, despite being in elementary school, but different people have different top levels of achievement and sometimes that level is below what any of us would like.