The article states:

    These rules are constraints on how to create your code
    and are only intended to be applied when doing katas or
    exercises.

These rules are an exercise to kick you out of an OOP mindset, not best practices for writing real code.

By that same token, I think it is a good rule when practicing OOP to not allow bare functions. It helps train yourself to think "Is there a logical class that this method belongs to?" In real code, of course, the answer is often "no". But when first learning OOP, it's good to teach yourself to ask that question in the first place.

Your code isn't a good example of "the OOP way", though. A fluent OOP solution would be:

    s = x.sin()

Another good rule when practicing OOP would be "No classes without state." That helps you avoid the habit of writing pointless "namespace" classes like the "Math" class you define here. Of course, again, sometimes it is useful to wrap a bunch of related behavior in a stateless class, but that should be the exception, not the default.