"(Harder.) If we add another morphism to the above example, it fails to be a category. Why? Hint: think about associativity of the composition operation."
One has to be careful with crowd-sourced wikipedia-like learning material. They aren't always helpful, although 80% of the time they are.
In this case, the 2 exercises probably belong right after the section on "Category Laws" but _before_ "Hask, the Haskell category."
That way, there's less confusion about what the exercises refer to and what they do NOT refer to. They certainly don't refer to Hask.
That said, the exercises presume acquaintance with partial orders / posets so there's some web-trawling to be done if the requirement is unmet.
And I'll also argue that the exercises aren't helpful. They may have their place in some musty math textbook. But for a general audience, it's like having some obscure concept (category) explained in terms of another obscure concept (partial order), with no-one getting any wiser.
p.s. if you are still stuck email me off-list for the answer.