That's a good question, and I understand how this can be confusing. It's true that an increase in the latent heat flux out of the ocean should lead to cooling of the ocean surface, not a warming. It's important to recognize that in the main "action center" of El Nino in the tropical Pacific, the heating is driven by the transport of ocean heat through the "Bjerknes feedback," as described in this post. The cooling influence of the increased evaporation (as well as that of the cloud shading) is a negative feedback that offsets the warming, but it's not strong enough to overcome the warming influence the Bjerknes feedback. Outside of the main Pacific action center, there isn't a significant change in evaporation to compete with the atmospheric warming that spreads away from the central-eastern Pacific.