However, given how low-level a language C++ is, we can actually break this assumption by setting i to y-x. Since &x[i] is the same as x+i, this means we are actually writing 23 to &y[0].

But that is undefined, you can't do x + (y - x) ie a pointer arithmetic that ends outside of bounds of an array. Since it is undefined, shouldn't C++ assume that changing x[..] can't change y[0]

edit: welp, if I read a few more lines into article I would see that it also tells it is undefined