If he suggested only using Kardia and nothing else, not even oGCDs, he was wrong.
Regardless of what he suggested though, you just implied that you rely on Diagnosis and that's the weakest of all your heals while also costing a GCD. The goal is generally to heal with your Abilities/ oGCDs first and only when that is definitely not enough you start using GCD heals. Kardia is effectively a free permanent regen as long as you keep dpsing so the less you drop dps for GCD healing, the better. Applies to all healers of course but SGE also loses Kardia heals when GCD healing.
So Haima/ Panhaima, Physis, Kera, Tauro, Druo etc. are your priority for healing.
Diagnosis is... really weak. Even sub 30, Kardia heals and Physis cover a large portion of the required healing.