The purpose of the RMD rule is to make you withdraw money from your retirement account over your own lifetime instead of leaving a large tax-deferred account to your beneficiaries. A traditional pension is based on your life expectancy ("your account was annuitized when you retired") and ends when you pass away, so it automatically meets the definition of a required distribution over your lifetime.
I don't know enough details of how the 1099-R works to know why some pensions get asked the RMD question and others don't, but if you are asked, it is perfectly ok to say that your payments do meet the requirements of the RMD rules.
