Nothing in this article is wrong. What adds to your knowledge of C is that certain operations on C pointers that would make sense if they were just bit pattern are actually undefined behavior. Undefined behavior may lead to subtle bugs due to how compilers do optimizations.

> I don't see how [. . .] is undefined behaviour.

This is a perfectly meaningless sentence. "Undefined behavior" doesn't mean "things which don't make sense in a particular implementation", it literally means behavior not defined by the spec. The spec declines to define the meaning of p==q when p and/or q is a freed pointer. There's nothing to "see" here: it's just not defined.

    #include <stdio.h>

    int main(void)
    {
        int x, y;
        int *p = &x + 1;
        int *q = &y;
        printf("%p %p %d\n", p, q, p == q);
        return 0;
    }

    0x7f7fffffdafc 0x7f7fffffdafc 0