I believe that ill health early retirement is discretionary rather than a right, and subject to medical assessment. Look at the scheme handbook for the guidelines on added years.
HR may have been referring to Cost Neutral Early Retirement? Obviously it's less financially beneficial to you, but it's a certainty that you'll actually get it.
I understand that IHER takes quite a while, so you'll be significantly closer to minimum retirement age when you get it, if you get it. That'll reduce the maximum potential number of additional years.
You can always apply for ill health early retirement first and if you don't get it you can then apply for CNER which you will get.