All programming languages - all computers even - have very clear rules. The complexity in software development usually has less to do with getting a computer to understand its own rules, and more to do with the boundaries between humans and those rules: for example, encoding human ideas into rigid computer rules, or trying as a human to understand the computer rules that someone else has written.

So when you say "if you know these rules nothing about dynamically typed code is inherently problematic", my intuition says "I probably don't know these rules". That goes for some of the basic rules like "the first argument to this function is the haystack, the second is the needle", but it also goes from some of the more complex rules like "functions that take a user ID can also take a user object and extract the ID from that" or "the allowed states for this FSM are X, Y, and Z, and are always written in capital letters". More importantly, a lot of the rules for my difference will not have been written by me, they'll have been written by my colleagues, or else they were written by me, but it was more than six months ago and my memory is a bit hazy on the details.

The point the previous poster was making, I think, was that while the rules may be very explicit (this is, after all, what any programming language is: explicit rules for a computer to follow), they can also be very complex. And, more specifically, the DefinitelyTyped examples show that the rules for dynamic software often tend to be very complex, or at least, complex enough to present difficulties when being modelled by a type system explicitly designed to model dynamic code.