The ocean isn’t acidifying, it is becoming slightly less basic. But that won’t do for scaremongering headlines.
Kw is also used as cologarithm pKw. If you are not aware, X = 10^(-pX), and pX = -log(X). So a lower pH is a greater concentration of [H+] (actually [H3O+]), a lower pOH is a greater concentration of [OH-], and a lower pKw is a greater self-ionization of water.
pKw decreases with increasing temperature. It decreases with increasing pressure. And it decreases with increasing ionic strength until around 0.6-0.8, then goes back up.
While those are held constant, a dropping pH does indicate acidification. pH + pOH = pKw . But if they are not constant, pOH can drop at the same rate as pH, because pKw is also dropping. As long as pH and pOH are both near pKw/2, the solution is neutral. The solution is only acidifying when pH is decreasing faster than pOH.
If you become slightly less basic, your pOH goes up. Under normal circumstances, that means pH is going down by an equal amount. Either way you describe it in English, de-alkalinization usually yields the same mathematical results as acidification. Deviation from this rule, which holds at standard temperature and pressure, and constant ionic strength, requires additional explanation.
pH is directly related to the concentration of H+ ions in solution, which is equivalent to being inversely correlated with the concentration of OH- ions in solution (a reduction in OH- ion concentration means a relative increase in H+ ion concentration).
Nobody cares about the absolute pH, except insofar as it affects wildlife and the ability of the oceans to provide protein that billions of people depend on to live.
And that is being affected, as noted in TFA.