Michael probably does realize that Roth is sick, and has been for years.

However that has obviously not prevented Roth from orchestrating several attempts on Michael's life, an attempted takeover of the Family, and a plan to land him in a federal prison.

By the time Michael says that he "has been dying from the same heart attack for 20 years", Hyman Roth is supposedly defeated, weakened, harmless, and looking for a place to live out his remaining days. What Michael is saying is that he really doesn't care how sick or old Roth is...he's still the enemy.