Short answer: Because EHBs must be covered and the ACA bans health-based pricing and exclusions, people with pre-existing conditions get the same comprehensive coverage at the same rates as everyone else.
Essential Health Benefits and the ACA’s consumer protections work together: insurers must cover all ten EHB categories, can’t deny coverage or charge more for a pre-existing condition, and can’t exclude care related to one. For someone with a chronic condition, that means guaranteed access to hospitalization, prescription drugs, mental-health care, and the other EHB categories at the same price a healthy person pays. It’s the combination of guaranteed issue, community rating, and EHBs that closed the old coverage gaps.