Solubility is defined as the concentration of the solute in a saturated solution. So when considering the increase in solubility with temperature, you have to check the enthalpy of solution of NaOH in a saturated solution of NaOH.

When you dissolve NaOH in pure water, the process is highly exothermic, but as the concentration of NaOH increases, the process becomes less exothermic and eventually near the saturation point it becomes endothermic (it becomes less favourable for more NaOH to dissolve).

So according to Le Chatelier’s Principle, although the dissolution process is overall exothermic, since solubility is determined only at the saturation point, the solubility increases with increase in temperature.