If you are not comfortable using the code, then don't use it.
I have a KLM Flying Blue membership, complete with printed card and membership number.
I have a reservation with National using a code that is listed as the KLM Flying Blue discount.
Many, many folks here on the Disboards have indicated they used the code with no problems at MCO. In fact, I don't think there has been one report of NOT being able to use the code.
So unless I can find a better rate using another code (for which I qualify), then this one still allows me to sleep at night. YMMV. If you don't feel okay with using the offer, then feel free to reserve elsewhere.