Coverage of pre-existing conditions usually depends on the date you were medically diagnosed with the condition. The insurance company determines that by doing a review of your medical records. For insurance purposes, you generally aren't considered to have a condition until it's documented in your personal medical record.
If there was never one word in your medical records about coronary artery disease or symptoms of coronary artery disease prior to a year ago, I think you have grounds for an appeal. Otherwise, insurance companies could deny coverage in this situation for pretty much anybody, because coronary artery plaque often starts building up at a young age.
The disability insurance company needs to explain to you your policy and the basis for their decision. If you still have a difference of opinion with them after that, then you may be able to get help from your human resources representative (if this coverage is a benefit offered by your employer) or your state insurance commissioner.