I'm thinking you didn't link the correct study. This study fully supports maintaining a balance to avoid limitation.
"4 Discussion
Our model of coupled N and P biogeochemical cycles in the ocean predicts that phytoplankton should perfectly regulate N and P concentrations in the upper layer because of its top‐down control on the upper, accessible nutrient pools. These results are in agreement with chemostat and resource‐ratios theories, which predict that organisms consume as much of the limiting resources as possible at equilibrium [e.g.,
Tilman,
1980;
Smith and Waltman,
1995]; as a result, they are expected to absorb any variation in the supply of a limiting nutrient in the accessible pool. "
They do show that this does change for deeper water which they define as not having light penetration.
"By contrast, we showed that variations in P and N supplies to the surface ocean impact nutrient concentrations in the deep, inaccessible layer. This occurs because biotic recycling of organic matter plays the role of a nutrient supply to the deep ocean, with the added complexity that the intensity of these biotic inflows depends on the biomass and N

ratio of phytoplankton in the upper layer."