[That human brains are Turing-complete] means nothing, although Greg Egan is quite impressed by it. Sad but true: Someone with an IQ of, say, 90 can be trained to operate a Turing machine, but will in all probability never understand matrix calculus.

It doesn't mean nothing; it means that people (like machines) can be taught to do things without understanding them.

(They can also be taught to understand, provided you reduce understanding to Turing-machine computations, which is harder. "Understanding that 1+1 = 2" is not the same thing as being able to output "2" to the query "1+1=".)