He died of pneumonia and had some sort of genetic defect that affected his heart
Effing, he didn't die of a drugs overdose but that doesn't mean drugs had no impact on his death. He had an extremely common heart condition which often has genetic roots. However it's extremely, extremely unusual for it to have developed so badly it can kill at 38. For that to happen you're going to have needed aggravating factors to make it worse. Like drug habits and extreme stress. His lungs were in a poor state due to smoking and drug abuse. He was abusing prescription drugs in the lead up to his death which would also have led to generally poor health.
For a 38 year old to die from the conditions is vanishingly rare in the healthy. There would almost always be aggravating factors which created the situation where they would kill someone that age. In Haim's case that was drug abuse.