Supremum is Coproduct in Order Category

Theorem
Let $\mathbf P$ be a poset category with ordering $\preceq$.

Let $p, q \in P_0$, and suppose they have some supremum $r = \sup \left\{{p, q}\right\}$.

Then $r$ is the coproduct of $p$ and $q$ in $\mathbf P$.

Proof
Let $\mathbf P^{\text{op}}$ be the dual category of $\mathbf P$.

From Dual of Poset Category, it is the poset category corresponding to the dual ordering $\succeq$.

From Dual Pairs (Order Theory), it follows that in $\mathbf P^{\text{op}}$:


 * $r = \inf \left\{{p, q}\right\}$

where $\inf$ denotes infimum.

By Infimum is Product in Poset Category, $r$ is the product of $p$ and $q$ in $\mathbf P^{\text{op}}$.

By Dual Pairs (Category Theory), $r$ is the coproduct of $p$ and $q$ in $\mathbf P$.

Also see

 * Infimum is Product in Poset Category, the dual result